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- Add the optional dedicated preference for the unsent log info.
- Save the unsent log info at each time the unsent log persisted.
- Read the unsent log info at next browser startup.
- Define the record method in UnsentLogStoreMetrics to record
  the count of the unsent samples, the count of the sent samples
  the percentage of unsent samples and the persisted logs size.

Bug: 1076564
Change-Id: I1ba3018ae819ff6f350fbf6a1fe8d7dbf0d17e26
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2224314
Commit-Queue: Tao Bai <michaelbai@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775841}