After the patches for bug 1543307 and bug 1543313 land, a11y clients will be able to traverse to and from OOP iframe documents, just as they can with iframes in the same content process. However, any win events fired from these documents (including descendants) are broken. This is because IAccessible::accChild fails for these ids.

This occurs because AccessibleWrap::GetRemoteIAccessibleFor only tries top level DocAccessibleParents; i.e. those at the top of the document hierarchy. It also needs to search documents at the top level of their content process; i.e. those for which DocAccessibleParent::IsTopLevelInContentProcess returns true.

It must be possible to retrieve any accessible by calling IAccessible::accChild on the RootAccessible (in the parent process) with the unique id of the desired accessible.
Among other things, this is the way accessibility events are targeted on Windows.
Previously, this code only searched remote documents at the top level of the tree.
In order to support out-of-process iframes, it must now also search embedded documents at the top level of their content process.

As part of this, the MSAA id must be set for out-of-process iframe documents, just as it is for top level documents.
This was already being sent from the content process, but previously, we didn't store this in the parent process.

Backed out 4 changesets (Bug 1553706, Bug 1543313) for failures in DocAccessibleChild.cpp

Push with failures: https://treeherder.mozilla.org/#/jobs?repo=autoland&resultStatus=testfailed%2Cbusted%2Cexception&revision=1a8bea02205d138e7881bcf63aff985ae1f2d64d&searchStr=browser%2Cchrome

https://treeherder.mozilla.org/#/jobs?repo=autoland&resultStatus=testfailed%2Cbusted%2Cexception&revision=fe21b958c55db9c5f6adfcf3ce96b732649a246e&searchStr=browser%2Cchrome

Failure logs: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=249302284&repo=autoland&lineNumber=2539
https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=249302225&repo=autoland&lineNumber=2793

Backout: https://hg.mozilla.org/integration/autoland/rev/523d45768463b8070a41fc606bdd9cbc3c352751

https://hg.mozilla.org/mozilla-central/rev/38c2478a4825