Handle layer location and transforms in repaint overlay

Previously repaint rects of all layers are combined into a single array.
If a layer has non-zero position or transform, the visualized repaint
rects will be incorrect.

Now convert layer tree into JSON, so that the overlay page script can
understand the whole layer tree.

BTW removed some original features that are not feasible with the new
repaint rect format. These features are also not very useful.

BUG=381221

Review URL: https://codereview.chromium.org/319073002

git-svn-id: svn://svn.chromium.org/blink/trunk@176651 bbb929c8-8fbe-4397-9dbb-9b2b20218538