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Metrics added in this CL:
1. SafeBrowsing.RT.HasValidCacheManager
This metrics is for checking if cache manager is valid when real time
url check is enabled.
2. SafeBrowsing.RT.GetCacheResult
This metrics is useful to identify the cache hit rate and the proportion
between safe verdicts and dangerous verdicts.
3. SafeBrowsing.RT.GetCache.Time
This metrics is useful to identify if the time to get cache is too long
compared with its effectiveness.

Bug: 1030989
Change-Id: I616ac3ad3bc207c907bd401801a09a9e46742818
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1974843
Commit-Queue: Xinghui Lu <xinghuilu@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726621}