Make squashing work with subpixel layout

This changes GraphicsLayerPaintInfo::offsetFromSquashingCLM
from an IntSize to a LayoutSize and changes
SquashingState::offsetFromAbsoluteForSquashingCLM from an
IntPoint to a LayoutPoint. This also adds a
subpixelAccumulation field to GraphicsLayerPaintInfo; this
gets computed at the same time as
GraphicsLayerPaintInfo::offsetFromRenderer.

BUG=327387

Review URL: https://codereview.chromium.org/143283011

git-svn-id: svn://svn.chromium.org/blink/trunk@169584 bbb929c8-8fbe-4397-9dbb-9b2b20218538