Implement UMA and internal data structure for tracking EWOULDBLOCK.

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 8e77773899ccf63bfd8f40942c601467ad2ea673..0ff0d0bc4afb93f098a4c7c785132c5cc8ba507b 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -38791,6 +38791,23 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="WebRTC.MaxApplicationConsecutiveBytesDiscard">
+  <owner>guoweis@chromium.org</owner>
+  <summary>
+    The maximum consecutive discarded bytes caused by not enough buffer
+    available in WebRTC's socket implementation.  This happens when WebRTC
+    IpcPacketSocket's throttling mechanism kicks in.
+  </summary>
+</histogram>
+
+<histogram name="WebRTC.MaxSystemConsecutiveDelayedBytes">
+  <owner>guoweis@chromium.org</owner>
+  <summary>
+    The maximum of consecutive delayed bytes caused by EWOULDBLOCKs from system.
+    This happens when system can't send any packet synchronously at that moment.
+  </summary>
+</histogram>
+
 <histogram name="WebRTC.NumDataChannelsPerPeerConnection">
   <owner>perkj@chromium.org</owner>
   <summary>
@@ -38802,6 +38819,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="WebRTC.PacketDiscardByApplication" units="%">
+  <owner>guoweis@chromium.org</owner>
+  <summary>
+    The percentage of packets discarded by WebRTC's socket layer due to
+    EWOULDBLOCKs when WebRTC IpcPacketSocket's throttling mechanism kicks in.
+  </summary>
+</histogram>
+
 <histogram name="WebRTC.PeerConnection.IPMetrics" enum="PeerConnectionCounters">
   <owner>mallinath@chromium.org</owner>
   <summary>
@@ -38897,6 +38922,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="WebRTC.SystemDelayedPackets" units="%">
+  <owner>guoweis@chromium.org</owner>
+  <summary>
+    The percentage of packets delayed due to ERR_IO_PENDING from system in a
+    WebRTC socket. This happens when system can't send any packet synchronously
+    at that moment.
+  </summary>
+</histogram>
+
 <histogram name="WebRTC.UnreliableDataChannelMessageSize" units="bytes">
   <owner>perkj@chromium.org</owner>
   <summary>
@@ -55130,6 +55164,13 @@ To add a new entry, add it with any value and run test to compute valid value.
   <affected-histogram name="Net.TCP_Connection_Latency_Interval"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="IPProtocolType" separator="_">
+  <suffix name="UDP"/>
+  <suffix name="TCP"/>
+  <affected-histogram name="WebRTC.MaxSystemConsecutiveDelayedBytes"/>
+  <affected-histogram name="WebRTC.SystemDelayedPackets"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="IPv6_Probe">
   <suffix name="IPv6_probe_skipped"
       label="with IPv6 not probed, and default OS settings used"/>