Add QuicSession.FailToSerializePacketLocation histogram.

This will be used to understand why exactly did packet serialization failed.

Change-Id: Idf12365e50301c07f8d4d82974ec80e2a160f339
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2419716
Auto-Submit: Renjie Tang <renjietang@chromium.org>
Commit-Queue: David Schinazi <dschinazi@chromium.org>
Reviewed-by: David Schinazi <dschinazi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808646}

tools/metrics/histograms/enums.xml

@@ -59939,6 +59939,32 @@ https://www.dmtf.org/sites/default/files/standards/documents/DSP0134_2.7.1.pdf
   <int value="168" label="SILENT_IDLE_TIMEOUT"/>
 </enum>
 
+<enum name="QuicFailToSerializePacketLocation">
+  <int value="0" label="kQuicFailToAppendPacketHeaderFastPath"/>
+  <int value="1" label="kQuicFailToAppendTypeFastPath"/>
+  <int value="2" label="kQuicFailToAppendStreamFrameFastPath"/>
+  <int value="3" label="kQuicFailToAddPaddingFastPath"/>
+  <int value="4" label="kQuicFailToWriteIetfLongHeaderLengthFastPath"/>
+  <int value="5" label="kQuicFailToEncryptPacketFastPath"/>
+  <int value="6" label="kQuicSerializePacketNonEmptyBuffer"/>
+  <int value="7" label="kQuicMissingInitialKey"/>
+  <int value="8" label="kQuicMissingHandshakeKey"/>
+  <int value="9" label="kQuicMissingZeroRttKey"/>
+  <int value="10" label="kQuicMissingOneRttKey"/>
+  <int value="11" label="kQuicFailToBuildPacketWithPaddingInitial"/>
+  <int value="12" label="kQuicFailToBuildPacketInitial"/>
+  <int value="13" label="kQuicFailToBuildPacketWithPaddingHandshake"/>
+  <int value="14" label="kQuicFailToBuildPacketHandshake"/>
+  <int value="15" label="kQuicFailToBuildPacketWithPaddingZeroRtt"/>
+  <int value="16" label="kQuicFailToBuildPacketZeroRtt"/>
+  <int value="17" label="kQuicFailToBuildPacketWithPaddingOneRtt"/>
+  <int value="18" label="kQuicFailToBuildPacketOneRtt"/>
+  <int value="19" label="kQuicFailToEncryptInitial"/>
+  <int value="20" label="kQuicFailToEncryptHandshake"/>
+  <int value="21" label="kQuicFailToEncryptZeroRtt"/>
+  <int value="22" label="kQuicFailToEncryptOneRtt"/>
+</enum>
+
 <enum name="QuicHandshakeFailureReason">
   <int value="0" label="UNKNOWN"/>
   <int value="1" label="BLACK_HOLE"/>

tools/metrics/histograms/histograms.xml

@@ -99486,6 +99486,15 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
+<histogram name="Net.QuicSession.FailToSerializePacketLocation"
+    enum="QuicFailToSerializePacketLocation" expires_after="2021-05-21">
+  <owner>renjietang@chromium.org</owner>
+  <owner>src/net/quic/OWNERS</owner>
+  <summary>
+    Record where exactly packet serialization fails for investigation.
+  </summary>
+</histogram>
+
 <histogram name="Net.QuicSession.FinalTcpCwnd" units="units"
     expires_after="2021-05-11">
   <owner>dschinazi@chromium.org</owner>