test_will_change.html was failed when WebRender was enabled. It was because nsDOMWindowUtils::CheckAndClearPaintedState() did not work when WebRender was enabled.

https://treeherder.mozilla.org/#/jobs?repo=try&revision=7125acb39241ff0c35f76a4904f0cbce1a6562a6&selectedJob=223695588

The CheckAndClearPaintedState() is used in the test_will_change.html.

https://searchfox.org/mozilla-central/rev/7d19d91f931be4e00a9f797913894dd3a8b961bc/layout/base/tests/chrome/test_will_change.html#46

The CheckAndClearPaintedState() uses nsIFrame::CheckAndClearPaintedState() and it depends on NS_FRAME_PAINTED_THEBES flag.

https://searchfox.org/mozilla-central/rev/3043e9d166182317ca92b54c0ceb6ab1f02b87ce/layout/generic/nsFrame.cpp#288

NS_FRAME_PAINTED_THEBES flag is used for checking if nsIFrame is painted, but WebRender normally does not do painting on content side. Then CheckAndClearPaintedState() could return false even when there was a paint.

For now, the followings set NS_FRAME_PAINTED_THEBES for WebRender

https://searchfox.org/mozilla-central/rev/0d6bfa48a2ae85d703c8ef40ab1cdd77193f09e8/gfx/layers/wr/WebRenderCommandsBuilder.cpp#375

https://searchfox.org/mozilla-central/rev/a21bea480d510274fd60888cef741b58858d78d9/gfx/layers/wr/WebRenderCommandBuilder.cpp#771

Simple fix is just skip test_will_change.html test, since content side basically does not do painting when WebRender is used.

With attachment 9039482 [details], the test was skipped with WebRender.
https://treeherder.mozilla.org/#/jobs?repo=try&revision=4a901d5835974f5f679ac06a11a964dc82be046c

https://hg.mozilla.org/mozilla-central/rev/7b7ff13f6be8