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This CL changes the accessory bar's top shadow to a "Toolbar Shadow" as
appropriate for a fixed piece of Chromium UI.
This would have the weird side effect of putting the accessory sheet on
the same perceived Z-level as the website. To prevent this, the
scrolling shadow changes to a smaller shadow that resembles the planned
"Modal shadow" which leverages the sheet above the website.

Screenshots and visual aids for this problem:
http://slides/1uto1gWimPsaU_3JVbNZqi7bk1tnsfVFV5Pgkck9cefM#slide=id.g482fab1c8c_0_265

Change-Id: I81451ac0605e601a4bd8ad6a511a2c2a050dc91c
Reviewed-on: https://chromium-review.googlesource.com/c/1348062Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Friedrich Horschig [CET] <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611124}