Adds UMA histogram metrics for SafeBrowsing PVer4 methods.

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 b93e169522784f1c2b94dbb1b83c298373b5469a..d213132c886790c2342cc776a5919cb93768f8e5 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -40201,6 +40201,20 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="SafeBrowsing.GetV4HashResult" enum="SB2GetHashResult">
+  <owner>kcarattini@chromium.org</owner>
+  <summary>
+    Track return status from V4 GetHash attempts (STATUS_200, STATUS_204,
+    NETWORK_ERROR, HTTP_ERROR, BACKOFF_ERROR), whether parsing a 200 result
+    failed (PARSE_ERROR), and dispensation of returned values (EMPTY, HIT,
+    MISS).  EMPTY means the response had no full hashes, and should contain all
+    of the 204 responses plus all *_ERROR cases.  HIT means that one of the full
+    hashes matched. MISS means that none of the hashes matched (there was a
+    prefix collision). (PARSE_ERROR, NETWORK_ERROR, HTTP_ERROR, and

[Nathan Parker, 2016/01/07 05:28:29]

Could remove the mention of past changes, since there is not v4 data back then. 
Also mention that this is similar to the GetHashResult, but for v4.

[kcarattini, 2016/01/07 06:06:13]

Done.

+    BACKOFF_ERROR were added in M36.) (MIN_WAIT_DURATION_ERROR added in M49).
+  </summary>
+</histogram>
+
 <histogram name="SafeBrowsing.ModuleBaseRelocation" units="BaseRelocationType">
   <owner>csharp@chromium.org</owner>
   <owner>krstnmnlsn@chromium.org</owner>
@@ -40266,6 +40280,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="SafeBrowsing.V4HttpResponseOrErrorCode"

[Nathan Parker, 2016/01/07 05:28:29]

Again I think this should be "Hash" specific.

[kcarattini, 2016/01/07 06:06:13]

Done.

+    enum="CombinedHttpResponseAndNetErrorCode">
+  <owner>kcarattini@google.com</owner>
+  <summary>
+    Response or error codes from the SafeBrowsing Pver4 service. Logged after a
+    GetHash or Update request finishes to capture the response code or error
+    code for that call.
+  </summary>
+</histogram>
+
 <histogram name="SB.BloomFilter" units="milliseconds">
   <obsolete>
     Has not been generated for years (7/8/14).
@@ -75540,6 +75564,7 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="6" label="NETWORK_ERROR"/>
   <int value="7" label="HTTP_ERROR"/>
   <int value="8" label="BACKOFF_ERROR"/>
+  <int value="9" label="MIN_WAIT_DURATION_ERROR"/>
 </enum>
 
 <enum name="SB2InterstitialAction" type="int">