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This CL adds a label when there are no targets in the sharesheet.
Waiting on mocks for label typography which will be updated in a
follow up CL.　This CL also switches main_view grid layout to boxlayout
for simplicity.

Bug: 1138037
Change-Id: Iebec778996c1db648a7fe7880f637dea554c6293
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2487743
Commit-Queue: Melissa Zhang <melzhang@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820144}