Add NetworkTimeTracker UMA histograms

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 c07da4196f443935a6db7e952a1ad62bdd322d08..d498d58e7d93bb972e9b70bed912739958ba27f2 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -34607,6 +34607,37 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="NetworkTimeTracker.UpdateTimeFetchAttempted">
+  <owner>estark@chromium.org</owner>
+  <owner>mab@chromium.org</owner>
+  <summary>
+    NetworkTimeTracker makes periodic queries to obtain a secure timestamp over
+    the network. This histogram increments whenever such a query is attempted.
+  </summary>
+</histogram>
+
+<histogram name="NetworkTimeTracker.UpdateTimeFetchFailed" enum="NetErrorCodes">
+  <owner>estark@chromium.org</owner>
+  <owner>mab@chromium.org</owner>
+  <summary>
+    NetworkTimeTracker makes periodic queries to obtain a secure timestamp over
+    the network. This histogram records the request's net error code whenever a
+    secure timestamp fetch results in a network error or a non-200 HTTP status
+    code.
+  </summary>
+</histogram>
+
+<histogram name="NetworkTimeTracker.UpdateTimeFetchValid">

[Steven Holte, 2016/07/28 19:22:46]

Add enum="BooleanValid"

[estark, 2016/07/28 20:30:49]

Done.

+  <owner>estark@chromium.org</owner>
+  <owner>mab@chromium.org</owner>
+  <summary>
+    NetworkTimeTracker makes periodic queries to obtain a secure timestamp over
+    the network. Whenever one of these requests results in a successful 200
+    status code, this histogram records true if the response parsed and
+    validated successfully, and false otherwise.
+  </summary>
+</histogram>
+
 <histogram name="NewTabPage.ActionAndroid" enum="NewTabPageActionAndroid">
   <owner>newt@chromium.org</owner>
   <summary>