Mainly cleanup required for session restore to work correctly with SHIP, which will fix some session restore test failures.

This reverts some of the work from bug 1507287. It tries to clean-up how
restores work when SHiP is enabled by splitting up the process into explicit
phases that both content and parent attempt to follow.

WIP: There's a few TODOs through the code, and probably a number of corner cases
that haven't been tested yet.

Depends on D95227

Based on :kashav's most recent try run (https://treeherder.mozilla.org/jobs?repo=try&revision=365c54434cf41e6a8e31ae3a89ad9de79388a774, based on comments from https://phabricator.services.mozilla.com/D94900#3103893), I took a quick look at the failures which were occuring. The first failure in the log was TEST-UNEXPECTED-FAIL | browser/components/sessionstore/test/browser_615394-SSWindowState_events_duplicateTab.js | undefined assertion name - Got +0, expected 1.

This assertion is being fired here: https://searchfox.org/mozilla-central/rev/277ab3925eae21b419167a34624ec0ab518d0c94/browser/components/sessionstore/test/browser_615394-SSWindowState_events_duplicateTab.js#46, which is caused by the SSWindowStateReady event not having fired before the SSTabRestoring event is fired. This makes sense with the changes in the revision, as SSWindowStateReady is fired from https://searchfox.org/mozilla-central/rev/277ab3925eae21b419167a34624ec0ab518d0c94/browser/components/sessionstore/SessionStore.jsm#4758.

Previously, this would have fired before the SSTabRestoring event, as that event would be fired from the receiveMessage callback from the content process after restoring session history is complete here: https://searchfox.org/mozilla-central/rev/277ab3925eae21b419167a34624ec0ab518d0c94/browser/components/sessionstore/SessionStore.jsm#1421, which wouldn't be called until at least one spin of the event loop after the history restore had been requested here: https://searchfox.org/mozilla-central/rev/277ab3925eae21b419167a34624ec0ab518d0c94/browser/components/sessionstore/SessionStore.jsm#4701-4706.

With kashav's changes, this now flipped around, as we can restore history synchronously, due to the history object being in the parent process. This means in turn that SSTabRestoring will fire before SSWindowStateReady, due to _sendRestoreHistory being called before _setWindowStateReady.

It may be enough to delay the call to SSTabRestoring by dispatching the call to _restoreHistoryComplete to the event loop.

I am uncertain whether this will fix any of the other test failures after the first one in that try push, and haven't looked into them at all.

ni? :jesup as requested by :neha

Randell is fixing the remaining test failures in Kashav's patches to get them ready for review and landing.

With mods from kashav's last Try run

Depends on D95227

Backed out 3 changesets (bug 1673617) for Browser-chrome failures in docshell/test/browser/browser_bug1347823.js. CLOSED TREE

Log:
https://treeherder.mozilla.org/logviewer?job_id=327176204&repo=autoland&lineNumber=2306

https://treeherder.mozilla.org/logviewer?job_id=327179045&repo=autoland&lineNumber=2837

Push with failures:
https://treeherder.mozilla.org/jobs?repo=autoland&group_state=expanded&revision=533092a71225cf694d48b2c5059ec7bdc2441187

Backout:
https://hg.mozilla.org/integration/autoland/rev/695669a8f1c6897ed0dc70a5f080487f8885df21

Backed out 3 changesetsfor causing failure on browser_child_resource.js.

Backout link: https://hg.mozilla.org/integration/autoland/rev/beccb3a1a3c4944d793975be9512145e4960666c

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&resultStatus=testfailed%2Cbusted%2Cexception%2Cusercancel%2Crunning%2Cpending%2Crunnable&revision=2a0dd70342ce2d2d32191cf3aaaf63398f6d0b7e&selectedTaskRun=D_w1ZZd0R8eB0tNMchkLiA.0&searchStr=Windows%2C7%2Copt%2CMochitests%2Ctest-windows7-32%2Fopt-mochitest-browser-chrome-e10s%2Cbc4

Failure log: https://treeherder.mozilla.org/logviewer?job_id=327553673&repo=autoland&lineNumber=4588
https://treeherder.mozilla.org/logviewer?job_id=327530999&repo=autoland&lineNumber=5693

https://hg.mozilla.org/mozilla-central/rev/56962291638a
https://hg.mozilla.org/mozilla-central/rev/ca97eba8c7c3
https://hg.mozilla.org/mozilla-central/rev/7c8497aa7523