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This CL adds support for importing and exporting sync metadata to
the BookmarkModelTypeProcessor.

Sync metadata are compiled into BookmarkModelMetadata proto. This
proto is used to communicate the sync metadata out and into the
processor.

This CL prepares for another CL that would actually persist/load
the sync metadata to/from disk.

Bug: 516866
Change-Id: I0a4af54383ac336a759c0504e4feee1ff1bda7c1
Reviewed-on: https://chromium-review.googlesource.com/1098918
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567982}