QUIC - Fix for crash in stable channel. Check for null |visitor_| before

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 5cc662505d7588bf3b7c04a170d4cc9f32eb2590..1e4ad82ee4ebf5aa47cc7e4d1d8b5738a60d4bf7 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -25676,6 +25676,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.QuicCloseConnection.NullVisitor" enum="BooleanNullVisitor">
+  <owner>rtenneti@chromium.org</owner>
+  <summary>
+    The number of times Connection's visitor is a nullptr when CloseConnection
+    is called.
+  </summary>
+</histogram>
+
 <histogram name="Net.QuicCryptoClientConfig.PopulatedFromCanonicalConfig"
     enum="BooleanPopulated">
   <owner>rtenneti@chromium.org</owner>
@@ -58277,6 +58285,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="1" label="Missing data in disk cache"/>
 </enum>
 
+<enum name="BooleanNullVisitor" type="int">
+  <int value="0" label="Connection's visitor is not a nullptr"/>
+  <int value="1" label="Connection's visitor is a nullptr"/>
+</enum>
+
 <enum name="BooleanOrphan" type="int">
   <int value="0" label="Non-orphan"/>
   <int value="1" label="Orphan"/>