https://www.chromestatus.com/features/5671480339726336

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/696f27afa62eb426545c29cefe0b0b43feeafe94

commit 696f27afa62eb426545c29cefe0b0b43feeafe94
Author: trchen <trchen@chromium.org>
Date: Wed Jun 22 03:40:23 2016

Force flattening for elements with non-initial value of opacity

As specified in latest editor's draft of [css-transforms], non-initial value
of opacity should force the used value transform-style to become flat.

Approved Intent to Implement and Ship:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/eBIp90_il1o

BUG= 617798 

Review-Url: https://codereview.chromium.org/2065233002
Cr-Commit-Position: refs/heads/master@{#401197}

[modify] https://crrev.com/696f27afa62eb426545c29cefe0b0b43feeafe94/third_party/WebKit/LayoutTests/compositing/geometry/preserve-3d-switching.html
[add] https://crrev.com/696f27afa62eb426545c29cefe0b0b43feeafe94/third_party/WebKit/LayoutTests/compositing/overflow-trumps-transform-style-expected.html
[delete] https://crrev.com/1d02ad2ac9c51df49c1402ec8deefeed66c25130/third_party/WebKit/LayoutTests/compositing/overflow-trumps-transform-style-expected.txt
[modify] https://crrev.com/696f27afa62eb426545c29cefe0b0b43feeafe94/third_party/WebKit/LayoutTests/compositing/overflow-trumps-transform-style.html
[modify] https://crrev.com/696f27afa62eb426545c29cefe0b0b43feeafe94/third_party/WebKit/LayoutTests/platform/android/compositing/geometry/preserve-3d-switching-expected.txt
[delete] https://crrev.com/1d02ad2ac9c51df49c1402ec8deefeed66c25130/third_party/WebKit/LayoutTests/platform/android/compositing/overflow-trumps-transform-style-expected.txt
[modify] https://crrev.com/696f27afa62eb426545c29cefe0b0b43feeafe94/third_party/WebKit/LayoutTests/platform/mac/compositing/geometry/preserve-3d-switching-expected.txt
[modify] https://crrev.com/696f27afa62eb426545c29cefe0b0b43feeafe94/third_party/WebKit/LayoutTests/platform/win/compositing/geometry/preserve-3d-switching-expected.txt
[add] https://crrev.com/696f27afa62eb426545c29cefe0b0b43feeafe94/third_party/WebKit/LayoutTests/transforms/opacity-forces-flatten-expected.html
[add] https://crrev.com/696f27afa62eb426545c29cefe0b0b43feeafe94/third_party/WebKit/LayoutTests/transforms/opacity-forces-flatten.html
[modify] https://crrev.com/696f27afa62eb426545c29cefe0b0b43feeafe94/third_party/WebKit/Source/core/css/resolver/StyleAdjuster.cpp
[modify] https://crrev.com/696f27afa62eb426545c29cefe0b0b43feeafe94/third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp
[modify] https://crrev.com/696f27afa62eb426545c29cefe0b0b43feeafe94/third_party/WebKit/Source/core/paint/PaintLayer.h
[modify] https://crrev.com/696f27afa62eb426545c29cefe0b0b43feeafe94/third_party/WebKit/Source/core/style/ComputedStyle.h