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Implemented the tab swithcer longpress menu on the bottom.
In the meanwhile, decreased the menu width to match the design spec.

Bug: 1011012
Change-Id: If1edc83bf9b10d332fc7627e9b88b2aed191a201
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1856729Reviewed-by: Theresa  <twellington@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Commit-Queue: Lijin Shen <lazzzis@google.com>
Cr-Commit-Position: refs/heads/master@{#711504}