Get rid of one-off code in ScrollingCoordinator to handle graphics layer offsets.

This allows us to remove a redundant squashing-related property in RenderLayer,
and is also a step towards not having non-compositing code know about how the layers
are positioned & drawn.

BUG=384520

Review URL: https://codereview.chromium.org/358913003

git-svn-id: svn://svn.chromium.org/blink/trunk@177133 bbb929c8-8fbe-4397-9dbb-9b2b20218538