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The call is implemented in the base class's implementation,
ModelTypeSyncBridge::DisableSync(). Without such a call, the
processor never takes care of deleting the metadata.

Because UserEventSyncBridge is a commit-only type (in fact the
only one), it forces initial_sync_done when loading metadata. This
is now moved to the processor, because it's common for all
commit-only types and it also needs to be taken care of if
DisableSync() is followed by enable-sync.

In order to test this, some refactoring of tests was needed (and I
chose to introduce MockModelTypeChangeProcessor), because:
a) The former tests had bugs (didn't actually verify some values,
   because callbacks not being run was treated as success)

b) The DisableSync() flow (currently) destroys and recreates the
   change processor, which requires some forwarding proxy to allow
   tests to verify the state across destructions of the processor.

Bug: 819233
Change-Id: I0f1524850ef65b795b7f7082ce4e85171b8e2898
Reviewed-on: https://chromium-review.googlesource.com/951604
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541805}