Partly correct vertical positioning of tab images against frame.

This computes the background tab image Y offset based on the refresh height of
the frame top, and simplifies the code by doing this unconditionally instead of
only when there's a tab background image.

This will not fix cases where a theme uses a custom toolbar image based on
assuming that there are 16 DIP above the top of tabs.  Added a TODO about that.

Bug: 866671
Change-Id: I9c9b5dacaba31f825d8392f5538ebd4cc8140367
Reviewed-on: https://chromium-review.googlesource.com/1147618
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Allen Bauer <kylixrd@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577780}
14 files changed