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The metadata will store additional developer-provided fields that will
help retain & recover the state of the background fetch.
Create/Get/Update registration database tasks are updated to support
the new metadata proto.

Bug: 826257, 757760
Change-Id: I5b97638d3ab90e482911fb50479e5b8c312b4331
Reviewed-on: https://chromium-review.googlesource.com/984356
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547669}