Invalidate scrollbars when window activity changes if needed

Non-overlay Mac scrollbars have different appearances for active vs inactive
windows, so pass activity changes down to Scrollbar, and have Scrollbar
invalidate itself if the ScrollbarTheme calls for it.

BUG=535984

Review URL: https://codereview.chromium.org/1424983002

Cr-Commit-Position: refs/heads/master@{#362630}
7 files changed