Fix pixel snapping issues when layers become composited

Within the render tree, locations and sizes are stored in LayoutUnits
and determining the final destination for an element involves walking
the render tree and accumulating the offset. When a layer becomes
composited, we start mid-way in the render tree, and don't accumulate
our offsets.

This change has us using LayoutUnits in RenderLayerBacking to store
our sub-pixel offset so our painting when composited pixel snaps the
same as when we aren't composited.

This patch is based on work Allan Sandfeld Jensen started on
https://bugs.webkit.org/show_bug.cgi?id=115304.

Review URL: https://codereview.chromium.org/20723003

git-svn-id: svn://svn.chromium.org/blink/trunk@164561 bbb929c8-8fbe-4397-9dbb-9b2b20218538