QUIC - Added a histogram to track number of times session_ is null when

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 4311dbb6de10e5c16068a5d047417c9d78d71971..f11c6afae937dbb6953913a4a5efdfb4515cc4a7 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -29032,6 +29032,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.QuicHttpStream::DoStreamRequest.NullSession"
+    enum="BooleanNullSession">

[rkaplow, 2016/06/20 20:19:18]

prefer instead of making a custom boolean enum, just use Boolean, and call
metric isNullSession

[ramant (doing other things), 2016/06/20 20:32:13]

Done.

+  <owner>rtenneti@chromium.org</owner>
+  <summary>
+    The number of times QuicHttpStream's session_ is a nullptr before
+    StartRequest() is called.
+  </summary>
+</histogram>
+
 <histogram name="Net.QuicInchoateClientHelloReason"
     enum="QuicServerConfigState">
   <owner>rtenneti@chromium.org</owner>
@@ -66107,6 +66116,11 @@ 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="BooleanNullSession" type="int">
+  <int value="0" label="Session is not a nullptr"/>
+  <int value="1" label="Session is a nullptr"/>
+</enum>
+
 <enum name="BooleanNullStream" type="int">
   <int value="0" label="Stream is not a nullptr"/>
   <int value="1" label="Stream is a nullptr"/>