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The previous logic was causing a memory corruption when aborting was not
invoked with the callback, but due to unmounting the file system.

As there are multiple paths for aborting, this CL adds a method IsAborted on
the Queue class so it's possible to check if a task has been aborted before.

TEST=unit_tests: *FileSystemProviderQueue*
BUG=454310

Review URL: https://codereview.chromium.org/968763002

Cr-Commit-Position: refs/heads/master@{#318652}