QUIC - check for null stream_ before calling stream_'s IsDoneReading

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 ba90135b713fcc1779b6921b501b8c09ecdcce27..94c92a35397b4ffea3acd0d7cff665310ffc8afd 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -26018,6 +26018,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.QuicReadAvailableData.NullStream" enum="BooleanNullStream">
+  <owner>rtenneti@chromium.org</owner>
+  <summary>
+    The number of times QuicHttpStream's stream_ is a nullptr before
+    IsDoneReading() is called.
+  </summary>
+</histogram>
+
 <histogram name="Net.QuicServerInfo.DiskCacheLoadTime" units="ms">
   <owner>rtenneti@chromium.org</owner>
   <summary>Time spent to load QUIC server information from disk cache.</summary>
@@ -58793,6 +58801,11 @@ 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="BooleanNullStream" type="int">
+  <int value="0" label="Stream is not a nullptr"/>
+  <int value="1" label="Stream is a nullptr"/>
+</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"/>