Add histogram to log whether a QuicGoAwayFrame is received for connection migration.

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 165d6fbf752c6c0bc52210b6f6b956282a3ce3da..f353f492096a1c04af9b33547a5a2390087065c2 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -26977,6 +26977,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.QuicSession.GoAwayReceivedForConnectionMigration"
+    enum="BooleanGoAwayForConnectionMigrationReceived">
+  <owner>zhongyi@chromium.org</owner>
+  <summary>
+    Whether QuicGoAwayFrame is received from server for connection migration due
+    to client's port change.

[Alexei Svitkine (slow), 2016/03/09 18:58:20]

Mention when it's logged.

[Zhongyi Shi, 2016/03/10 00:42:22]

Done.

+  </summary>
+</histogram>
+
 <histogram name="Net.QuicSession.HandshakeConfirmedTime" units="Milliseconds">
   <owner>rch@chromium.org</owner>
   <summary>
@@ -60198,6 +60207,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="1" label="WebView-based flow"/>
 </enum>
 
+<enum name="BooleanGoAwayForConnectionMigrationReceived" type="int">

[Alexei Svitkine (slow), 2016/03/09 18:58:20]

Nit: If the values are just the ones below, you can use BooleanReceived instead
of defining the new enum.

[Zhongyi Shi, 2016/03/10 00:42:22]

Done.

+  <int value="0" label="Not received"/>
+  <int value="1" label="Received"/>
+</enum>
+
 <enum name="BooleanHadAddress" type="int">
   <int value="0" label="Did not have address."/>
   <int value="1" label="Had address."/>