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This CL adds proper margins for SharesheetTargetButton and
updates bubble targets to have a fixed size and alignment.
We also increase the size of the bubble to a fixed width.

Bug: 1097623
Change-Id: I9f236a6dd73dbf1a9159d8e6a8ee38a4447c06aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2336117Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Melissa Zhang <melzhang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794854}