[SPDY] Add histograms for flow control stalls

tools/metrics/histograms/histograms.xml

 <!--
 Copyright 2013 The Chromium Authors. All rights reserved.
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 -->
 
 <!--
 This file is used to generate a comprehensive list of Chrome histograms along
 with a detailed description for each histogram.
 
@@ skipping 1911 matching lines @@
   </obsolete>
   <summary>
     The counts of the type of sockets returned by the SOCKS pool.
   </summary>
 </histogram>
 
 <histogram name="Net.SpdyConnectionLatency" units="milliseconds">
   <summary>Time from when the Connect() starts until it completes.</summary>
 </histogram>
 
+<histogram name="Net.SpdyDataFrameSendStalledBySession" enum="BooleanSuccess">
+  <summary>
+    Whether an attempt to send a data frame was stalled by the session flow

[jar (doing other things), 2013/04/19 06:22:29]

I'm not sure whether the enum value "success" means "stalled" or "not".

IMO, you'd be much clearer defining a custom enum, which was simply "stalled
send" "successful send."

Maybe I'm being thick.... but it is generally really nice when the label really
tells you what the bucket means.

[akalin, 2013/04/19 08:21:15]

Sounds good to me -- I wasn't sure of the best practices re. custom enums in
histograms. How's this? (I changed the semantics of the enums slightly.)

+    control send window.
+  </summary>
+</histogram>
+
+<histogram name="Net.SpdyDataFrameSendStalledByStream" enum="BooleanSuccess">
+  <summary>
+    Whether an attempt to send a data frame was stalled by the stream flow
+    control send window.
+  </summary>
+</histogram>
+
 <histogram name="Net.SpdyIPPoolDomainMatch" enum="SpdyIPPoolDomainMatch"
     units="count">
   <summary>
     Status of checking if a SPDY domain can handle a IP match.  If a match is
     found, we successfully used the IP Pooling.  If a match is not found, we
     could have used IP Pooling, except the TLS Cert didn't match the IP-pooled
     domain.
   </summary>
 </histogram>
 
@@ skipping 3654 matching lines @@
 <fieldtrial name="SSLResumption">
   <group name="Resume_Handshake" label="Session Resumption"/>
   <group name="Full_Handshake" label="Full"/>
   <affected-histogram name="Net.SSL_Connection_Latency"/>
   <affected-histogram name="Net.SSL_Connection_Latency_Google"/>
 </fieldtrial>
 
 </fieldtrials>
 
 </histogram-configuration>