[Sharesheet] Clean up is_bubble_closing_ check so it won't be missed.

Fixing a bug where the is_bubble_closing_ condition check was missed
in OnAcceleratorPressed by moving the check into
CloseWidgetWithAnimateFadeOut. This ensures that we never try to close
the bubble a second time when it's already in the process of closing.

Bug: 1316990
Change-Id: I2dad22aa0182f800b243417d32607bc5242f2b0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3609731
Reviewed-by: Tim Sergeant <tsergeant@chromium.org>
Commit-Queue: Melissa Zhang <melzhang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#996998}