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Add drive sync handler externs file to the Closure build, and "fix" up
Closure compile issues. Make DriveSyncHandler an interface and use new
type DriveSyncHandlerImpl for the concrete implementation. Add a BUILD
rule for mock_drive_sync_handler and fix Closure compile issues.

Actions model unittest defined MockDriveSyncHandler methods.  Move the
methods to mock_drive_sync_handler and update the actions model test.

Bug: 908045
Change-Id: I6b150f13f47e3b6263b16a1faefa0a1342cb36bb
Reviewed-on: https://chromium-review.googlesource.com/c/1349509Reviewed-by: Stuart Langley <slangley@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610747}