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This CL adds the convert functions to convert the restore data to
base::Value.

Add FullRestoreFileHandler to convert the restore data to JSON string,
and write to the full restore file.

BUG=1146900

Change-Id: I1e93579464cf97ab04235599fc5519f86a606e15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2581709
Commit-Queue: Nancy Wang <nancylingwang@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836549}