Migrates QUIC sessions to a new network when old network is (about to be) disconnected.

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 aa1c6849158787613595293b12140d7873be1300..37daa3930ed2ec1c60946151ebc8c0615d65b097 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -25591,6 +25591,12 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.QuicSession.ConnectionMigration"
+    enum="QuicConnectionMigrationStatus">
+  <owner>jri@chromium.org</owner>
+  <summary>The result of a QUIC connection migration attempt.</summary>
+</histogram>
+
 <histogram name="Net.QuicSession.ConnectionTypeFromPeer" enum="AddressFamily">
   <owner>rch@chromium.org</owner>
   <summary>
@@ -74370,6 +74376,14 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="5" label="FIVE_PACKETS_LOST"/>
 </enum>
 
+<enum name="QuicConnectionMigrationStatus" type="int">
+  <int value="1" label="NO_MIGRATABLE_STREAMS"/>
+  <int value="2" label="ALREADY_MIGRATED"/>
+  <int value="3" label="INTERNAL_ERROR"/>
+  <int value="4" label="TOO_MANY_CHANGES"/>
+  <int value="5" label="SUCCESS"/>
+</enum>
+
 <enum name="QuicDisabledReason" type="int">
   <int value="1" label="Public reset post handshake"/>
   <int value="2" label="Timeout with open streams"/>