Introduce UMA metrics for path MTU discovery.

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 1b4caef7d0006bd268cc1ffde877abd44eb9213c..bcbdd85306fccd797c92076f6115a5d64dbae6a3 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -22117,6 +22117,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.QuicSession.ClientSideMtu" units="bytes">
+  <owner>rch@chromium.org</owner>
+  <summary>
+    The final maximum packet size of the connection used by the client before
+    the session is closed.
+  </summary>
+</histogram>
+
 <histogram name="Net.QuicSession.ClosedDuringInitializeSession" enum="Boolean">
   <owner>rch@chromium.org</owner>
   <summary>
@@ -22369,6 +22377,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.QuicSession.MtuProbesSent">
+  <owner>rch@chromium.org</owner>
+  <summary>
+    The number of MTU probes sent by the client during the session.  Logged when
+    the connection is destroyed.
+  </summary>
+</histogram>
+
 <histogram name="Net.QuicSession.NumOpenStreams">
   <owner>rch@chromium.org</owner>
   <summary>
@@ -22523,6 +22539,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.QuicSession.ServerSideMtu" units="bytes">
+  <owner>rch@chromium.org</owner>
+  <summary>
+    The largest packet which the client received from the server during the
+    session.
+  </summary>
+</histogram>
+
 <histogram name="Net.QuicSession.StreamFlowControlBlocked"
     enum="BooleanBlocked">
   <owner>rtenneti@chromium.org</owner>