Add a new histogram for migration before handshake to track the status.

This histogram will track how often the migration before handshake ends
up with success, and how often is not.

Bug: 790547
Change-Id: I8cb9c3dbcbb7262955c69284f1a12d2e11be61eb
Reviewed-on: https://chromium-review.googlesource.com/c/1315933
Commit-Queue: Zhongyi Shi <zhongyi@chromium.org>
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605453}

net/quic/quic_stream_factory.cc

@@ -795,6 +795,12 @@ int QuicStreamFactory::Job::DoConfirmConnection(int rv) {
     }
   }
 
+  if (network_ != NetworkChangeNotifier::kInvalidNetworkHandle &&
+      network_ != factory_->default_network()) {
+    UMA_HISTOGRAM_BOOLEAN("Net.QuicStreamFactory.MigrationBeforeHandshake",
+                          rv == OK);
+  }
+
   if (rv != OK)
     return rv;
 

tools/metrics/histograms/histograms.xml

@@ -59538,6 +59538,15 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="Net.QuicStreamFactory.MigrationBeforeHandshake"
+    enum="BooleanSuccess" expires_after="2020-11-01">
+  <owner>zhongyi@chromium.org</owner>
+  <summary>
+    Status of the connection which is created for migration before handshake is
+    confirmed.
+  </summary>
+</histogram>
+
 <histogram name="Net.QuicStreamFactory.PublicResetsPostHandshake"
     units="resets">
   <owner>ckrasic@chromium.org</owner>