QUIC - add new UMA histograms to help understand HPACK head of line blocking.

net/quic/quic_chromium_client_session.cc

Index: net/quic/quic_chromium_client_session.cc
diff --git a/net/quic/quic_chromium_client_session.cc b/net/quic/quic_chromium_client_session.cc
index 91f763c9d6dcdf6b4186589fe9adbb173342e40d..34936c322cfdbea88ddbb0a748b1ee1508daa175 100644
--- a/net/quic/quic_chromium_client_session.cc
+++ b/net/quic/quic_chromium_client_session.cc
@@ -143,6 +143,22 @@ std::unique_ptr<base::Value> NetLogQuicPushPromiseReceivedCallback(
   return std::move(dict);
 }
 
+class HpackEncoderDebugVisitor : public QuicHeadersStream::HpackDebugVisitor {
+  void OnUseEntry(QuicTime::Delta elapsed) override {
+    UMA_HISTOGRAM_TIMES(
+        "Net.QuicHpackEncoderIndexedEntryAge",

[Alexei Svitkine (slow), 2016/05/24 18:00:30]

Nit: Maybe slightly more readable if you add another . in between:

Net.QuicHpackEncoder.IndexedEntryAge

If you make the change, also change the one below and histograms.xml.

[Buck, 2016/05/24 18:15:06]

Done.

+        base::TimeDelta::FromMicroseconds(elapsed.ToMicroseconds()));
+  }
+};
+
+class HpackDecoderDebugVisitor : public QuicHeadersStream::HpackDebugVisitor {
+  void OnUseEntry(QuicTime::Delta elapsed) override {
+    UMA_HISTOGRAM_TIMES(
+        "Net.QuicHpackDecoderIndexedEntryAge",
+        base::TimeDelta::FromMicroseconds(elapsed.ToMicroseconds()));
+  }
+};
+
 }  // namespace
 
 QuicChromiumClientSession::StreamRequest::StreamRequest() : stream_(nullptr) {}
@@ -368,6 +384,14 @@ QuicChromiumClientSession::~QuicChromiumClientSession() {
       static_cast<base::HistogramBase::Sample>(stats.max_sequence_reordering));
 }
 
+void QuicChromiumClientSession::Initialize() {
+  QuicClientSessionBase::Initialize();
+  headers_stream()->SetHpackEncoderDebugVisitor(
+      base::WrapUnique(new HpackEncoderDebugVisitor()));
+  headers_stream()->SetHpackDecoderDebugVisitor(
+      base::WrapUnique(new HpackDecoderDebugVisitor()));
+}
+
 void QuicChromiumClientSession::OnHeadersHeadOfLineBlocking(
     QuicTime::Delta delta) {
   UMA_HISTOGRAM_TIMES(