[omnibox] Animate tab switch button size changes

The tab switch button only exists at quantized sizes, depending on
window width. Currently it jumps between those sizes. We would prefer
that it animate between them. This CL adds the beginning of animation,
putting off issues like delaying animation until window shrinking
stops, etc.

The CL also changes the ellision type to fade.

Bug: 780835
Change-Id: Ic4a9997e213bae5415d73a7f83b410b788a95279
Reviewed-on: https://chromium-review.googlesource.com/1067667
Commit-Queue: Kevin Bailey <krb@chromium.org>
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562928}
4 files changed