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This change adds a check to see if a user's Google Drive directory is
mounted on Chrome OS. If Drive is not mounted then we don't show the
Save to Drive option in the dropdown.

Reasons why Drive could not be mounted (callback would return false)
include: Drive is disabled by the user, disabled by enterprise policy,
or is in a crashing state.

Also remove "isDriveMounted" as a print preview handler message callback
because this data is now sent back with the initial settings.

Bug: 1112416
Change-Id: I26acf17a41cb178f99c630b2daf43d02ae08134d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2382204
Commit-Queue: Gavin Williams <gavinwill@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803341}