Fix subpixel accumulation for composited content layers

Fix subpixel accumulation for composited content layers

The content box rect was improperly snapped in the local space. We should have
snapped in the layout space instead, including subpixel accumulation.

BUG=627683

[trchen, 2016-07-21 00:07:04]

trchen@chromium.org changed reviewers:
+ chrishtr@chromium.org

[trchen, 2016-07-21 02:07:47]

The CQ bit was checked by trchen@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-07-21 02:08:09]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-07-21 04:09:51]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-07-21 04:09:53]

Dry run: Try jobs failed on following builders:
  linux_chromium_rel_ng on master.tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[chrishtr, 2016-07-21 18:02:31]

This CL needs a new test that exercises what's in the bug.

https://codereview.chromium.org/2172503002/diff/20001/third_party/WebKit/Layo...
File third_party/WebKit/LayoutTests/TestExpectations (right):

https://codereview.chromium.org/2172503002/diff/20001/third_party/WebKit/Layo...
third_party/WebKit/LayoutTests/TestExpectations:1358: crbug.com/627683
virtual/gpu/fast/canvas/canvas-toDataURL-webp.html [ NeedsRebaseline ]
Please link to bot results on this so I can see the change. Or upload the change
on one platform.

https://codereview.chromium.org/2172503002/diff/20001/third_party/WebKit/Sour...
File third_party/WebKit/Source/core/layout/LayoutTextTrackContainer.cpp (left):

https://codereview.chromium.org/2172503002/diff/20001/third_party/WebKit/Sour...
third_party/WebKit/Source/core/layout/LayoutTextTrackContainer.cpp:62: IntSize
videoSize = videoLayoutObject.videoBox().size();
Why the changes here? videoBox() has integer size.

https://codereview.chromium.org/2172503002/diff/20001/third_party/WebKit/Sour...
File
third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
(right):

https://codereview.chromium.org/2172503002/diff/20001/third_party/WebKit/Sour...
third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp:93:
// FrameView are somehow pre-snapped. This is to match behavior of
LayoutPart::updateWidgetGeometry().
PartPainter::paintContents does rounded snapping that looks the same as this.
This is now the third place that this code appears.
Please refactor to have it centralized in a method on LayoutPart.

https://codereview.chromium.org/2172503002/diff/20001/third_party/WebKit/Sour...
third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp:354:
innerCompositor->frameViewDidChangeLocation(roundedIntPoint(contentsBox().location()));
Yet another instance of frame rounding to be refactored?

https://codereview.chromium.org/2172503002/diff/20001/third_party/WebKit/Sour...
third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp:1988:
return m_owningLayer.subpixelAccumulation() -
LayoutSize(m_graphicsLayer->offsetFromLayoutObject());
why subpixelAccumulation - offsetFromLayoutObject? Why not:

-(offsetFromLayoutObject + subpixelAccumulation)