Reattach video layer to a higher hierarchy for overlay fullscren video

The current implementation reattaches video layer under the RenderView which
is a descendant of the scroll layer. In usual situation it is okay because
fullscreen elements are fixed positioned and compensated with scroll position.
For videos in an <iframe>, the layer can be wrongfully positioned because the
difference in scroll position.

This CL changes the video layer to reattach to under the root layer, and always
reset layer position to (0, 0).

BUG=362504

Review URL: https://codereview.chromium.org/273203003

git-svn-id: svn://svn.chromium.org/blink/trunk@173918 bbb929c8-8fbe-4397-9dbb-9b2b20218538