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Add the following UMA metrics:
- Search.HistoryReport.DeltaFile.LastSeqNo
- Search.HistoryReport.DeltaFile.LevelDBEntries
- Search.HistoryReport.UsageReportsBuffer.LevelDBEntries

These are health checking signals to see if Icing-related levelDBs
could grow too big.

Bug: 857345
Change-Id: Icfc67b3442864a1c31b15427de40b406ea674ff7
Reviewed-on: https://chromium-review.googlesource.com/1117937Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573559}