Make ScrollbarThemeAura selectively invalidate scrollbar parts.

In particular:
- when hovering or pressing the thumb, repaint only the thumb
- when thumb position changes, repaint the track only if it changes
  (e.g. due to a button being disabled upon reaching the end)

BUG=549277

Review URL: https://codereview.chromium.org/1525803002

Cr-Commit-Position: refs/heads/master@{#367082}
13 files changed