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While Drive is mounting, a fake Google Drive entry is shown in Files
app. If mounting fails, begin returning false for the is drive enabled
"pref" to Files app and dispatch an event notifying it about the change
in prefs.

Bug: 889732
Change-Id: Ia79c19a36904bb73f7a3a8ed86ab52dd685689dc
Reviewed-on: https://chromium-review.googlesource.com/c/1312442
Commit-Queue: Sam McNally <sammc@chromium.org>
Reviewed-by: Sergei Datsenko <dats@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604557}