Calculate paint invalidation rect for scrollbars

Remove eager invalidation of scrollbars except when they are removed.
Invalidate scrollbars during paint invalidation so that we can calculate
paint invalidation rects for them. The rects are required by
PaintController to calculate display item bounds.

BUG=529938

Review URL: https://codereview.chromium.org/1406133005

Cr-Commit-Position: refs/heads/master@{#360144}
79 files changed