[SPDY] Add histograms for flow control stalls

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 e07bf1d13be3523f174466fa9a88debdd449bc25..8f4036a17124769e998df7bb14f7851b0340d073 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -1929,6 +1929,20 @@ other types of suffix sets.
   <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>