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Other varieties of the toobar depend on the shadow drawable not
changing. This change reverts the bottom_toolbar_shadow to its
original state (crrev.com/c/646428) and creates a separate one for
Chrome Modern.

BUG=

Change-Id: I3b7964cf603df1308a00942cd728726d5268f71c
Reviewed-on: https://chromium-review.googlesource.com/660984
Commit-Queue: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501332}