Pass accurate contentsScale to LayerImpl.

This is the first part of fixing fuzzy content issue in composited layers in
scaled pages.

There are two reasons of fuzzy content:

1) Inaccurate scale: The scale in transformation is calculated with
     contentBounds.width / bounds.width
     contentBounds.height / bounds.height
   in layer_tree_host_common.cc and other places.
   However, contentBounds is a IntSize which is calculated from bounds
   and contentsScale in layer.cc:
     IntRect(lroundf(bounds.width * contentsScale),
             lroundf(bounds.height * contentsScale))
   This causes the scale like 0.993 or 1.007 in the drawTransformation etc.
   where identity transformation is expected.

   To resolve the issue, instead of calculating scale using contentBounds,
   pass the accurate contentsScale from Layer to LayerImpl.

2) Pixel on surfaces are not aligned. Will describe this in the CL for the
   second part.

(See https://bugs.webkit.org/show_bug.cgi?id=84187 for more details).

Change-Id: I8f59f0460e1b212223e2c8c551b4127d8239e5cc

BUG=bugs.webkit.org/show_bug.cgi?id=84187
TEST=ContentsScalingLayerTest.checkContentBounds, LayerTreeHostCommonTest.verifyLayerTransformsInHighDPIAccurateScaleZeroPosition, LayerTreeHostCommonTest.verifyRenderSurfaceTransformsInHighDPIAccurateScaleZeroPosition


Review URL: https://chromiumcodereview.appspot.com/11276060

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@165269 0039d316-1c4b-4281-b951-d872f2087c98

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