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This CL fixes the button highlight when toolbar buttons are focused
using a keyboard. There are two unrelated fixes:
1. ToggleTabStackButton now inherits ChromeImageButton instead of
   generic ImageButton.
2. ToolbarMenuButtonPhone style no longer forces the background to be
   null, keeping selectableItemBackground set by ToolbarButton style.

Bug: 955037
Change-Id: Idc20378c9660dc0d95189af5efc417dd861512da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1725909
Commit-Queue: Theresa <twellington@chromium.org>
Auto-Submit: Boris Sazonov <bsazonov@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682259}