PVer4: Add UMA metrics for time taken to: load DB, do prefix check,

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 6985dada5c972a922c69887a6263e1836bb7eaae..8ebd5a1cf343da0d11e22508793b60022a6e641e 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -51741,6 +51741,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="SafeBrowsing.V4DatabaseOpen.Time" units="ms">
+  <owner>vakh@chromium.org</owner>
+  <summary>
+    The time it takes to parse and load the SafeBrowsing database from disk, in
+    milliseconds.
+  </summary>
+</histogram>
+
 <histogram name="SafeBrowsing.V4DecodeAdditionsResult"
     enum="SafeBrowsingV4DecodeResult">
   <obsolete>
@@ -51803,6 +51811,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>Track get hash response hits for V4 full hash requests.</summary>
 </histogram>
 
+<histogram name="SafeBrowsing.V4GetPrefixMatches.Time" units="ms">
+  <owner>vakh@chromium.org</owner>
+  <summary>
+    The time that it took to check a URL against our in-memory database.

[Nathan Parker, 2016/10/22 00:24:55]

Maybe note that this is the sum of all in-memory checks, but dominated by
CheckBrowseUrl.

[vakh (use Gerrit instead), 2016/10/22 01:34:34]

Done.

+  </summary>
+</histogram>
+
 <histogram name="SafeBrowsing.V4MergeUpdateTime" units="ms">
   <obsolete>
     Replaced by SafeBrowsing.V4*.MergeUpdate.Time.