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PVer4: Add UMA metrics for time taken to: load DB, do prefix check, and network time to do full hash check

SB2.DatabaseOpen: The time it takes to read, parse, and load the database from disk into memory.
SB2.FilterCheck: The time it takes to check the prefix hashes for a match. I tested a known phishing URL.
SB2.Network: The time it takes to get the full hash response from the server and parse it.

This is anecdotal, but I ran the two DB managers in parallel mode 5 times (5 different launches of Chrome) and got the following results:
+ SB2.DatabaseOpen is consistently faster for PVer4 by 59% (0.0919344s vs 0.0375474s). The PVer4 version is called: SafeBrowsing.V4DatabaseOpen.Time

+ SB2.FilterCheck is consistently faster for PVer4 by 30% (0.0093838s vs 0.00659247s). The PVer4 version is called: SafeBrowsing.V4GetPrefixMatches.Time

+ SB2.Network is consistently *slower* for PVer4 by 4.94% (0.2113358s vs 0.221783s). -- This slowness needs to be investigated but is somewhat acceptable since this is only used for 1/1000 hash checks. The PVer4 version is called: SafeBrowsing.V4GetHashNetwork.Time

Adding these metrics would allow us to do a more reasonable comparison.

BUG=543161

Review-Url: https://chromiumcodereview.appspot.com/2441923003
Cr-Commit-Position: refs/heads/master@{#426985}