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This CL resets the unique pointer to the menu model adapter which runs
the multipaste menu. Note that the destruction is called asynchronously.
Because the callback of closing the menu is called from MenuRunnerImpl.
Meanwhile, MenuRunnerImpl lives in the menu model adapter. So it is
risky to destruct the menu model adapter in a synchronous way, although
it works fine right now.

Bug: 1148591
Change-Id: Ifdd2d526d74b60090eb495cdc3c0309d475e4f66
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2536850Reviewed-by: David Black <dmblack@google.com>
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827138}