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This CL adds a feature flag for enabling the sharesheet on desktop
platforms.

Bug: 1097623
Change-Id: I5a99db0ed041b3669046a900ad3bedfbbb3f57fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2256941Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Melissa Zhang <melzhang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781673}