Pushed by dmu@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/87219897a280 Enable VR process in Nightly Windows. r=kip

Backed out changeset 87219897a280 (Bug 1516554) for reftest failures on draw_rect.html and wpt assertion failures. Backout: https://hg.mozilla.org/integration/autoland/rev/f000153a1f7f599af8c805c5d447ef81fc5f14d3 Push with failures: https://treeherder.mozilla.org/#/jobs?repo=autoland&resultStatus=pending%2Crunning%2Csuccess%2Ctestfailed%2Cbusted%2Cexception&selectedJob=220035541&revision=87219897a280cfc756aa1cf883324512285fdd74 Reftest failure log: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=220035541&repo=autoland&lineNumber=18029 WPT failures log: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=220035277&repo=autoland&lineNumber=62640 https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=220038119&repo=autoland&lineNumber=7290

Pushed by dmu@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/115387297036 Enable VR process in Nightly Windows. r=kip https://hg.mozilla.org/integration/autoland/rev/8617fed7a3e0 Part 2: Fix nsTSubstring assertion when looking for Oculus VR lib. r=kip https://hg.mozilla.org/integration/autoland/rev/625005dcd0b3 Part 3: Fix VRGPUParent memory leak in the destructor. r=kip https://hg.mozilla.org/integration/autoland/rev/bff1af3da433 Part 4: Fix Win 7 try fails when VR process enable but no GPU process. r=kip https://hg.mozilla.org/integration/autoland/rev/518dbd08c0d7 Part 5: Allow VR using the existing shmem. r=kip https://hg.mozilla.org/integration/autoland/rev/ffe71a987b82 Part 6: Remove BackgroundHangMonitor in VRProcessChild because we will init it by XPCOM. r=kip https://hg.mozilla.org/integration/autoland/rev/829462dca352 Part 7: Resolve MessageLoop destroy before VR IPC channels close issue. r=kip