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This CL adds a temporary button to the toolbar that will be used to
trigger the sharesheet during development.

Bug: 1097623
Change-Id: Icb9029fbd508e6e358bd82381886dbd7b6050cae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2262400Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Melissa Zhang <melzhang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784312}