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..b8f1e165adc0fb2ed87141d8ecf4f6e4c4fe171d 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -34607,6 +34607,49 @@ 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 a net error code whenever a secure
+    timestamp fetch fails.
+  </summary>
+</histogram>
+
+<histogram name="NetworkTimeTracker.UpdateTimeFetchSucceeded">
+  <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 an HTTP request to retrieve
+    a timestamp results in a successful 200 status code. Note that, unlike the
+    NetworkTimeTracker.UpdateTimeFetchValid histogram, this histogram only
+    records when an HTTP request succeeded and does not record any validation of
+    the received data.
+  </summary>
+</histogram>
+
+<histogram name="NetworkTimeTracker.UpdateTimeFetchValid">
+  <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 an HTTP request to retrieve
+    a timestamp results in a successful 200 status code and the response is
+    valid (properly signed, and is valid JSON with the expected structure).
+  </summary>
+</histogram>
+
 <histogram name="NewTabPage.ActionAndroid" enum="NewTabPageActionAndroid">
   <owner>newt@chromium.org</owner>
   <summary>