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 6ae42dd3755639aff100fd7b48968765d559c204..d7bebcbc18f9ddaa7631d7f3785e5cc895fb0b59 100644
--- a/net/quic/quic_chromium_client_session.cc
+++ b/net/quic/quic_chromium_client_session.cc
@@ -141,6 +141,22 @@ 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.QuicHpackEncoder.IndexedEntryAge",
+        base::TimeDelta::FromMicroseconds(elapsed.ToMicroseconds()));
+  }
+};
+
+class HpackDecoderDebugVisitor : public QuicHeadersStream::HpackDebugVisitor {
+  void OnUseEntry(QuicTime::Delta elapsed) override {
+    UMA_HISTOGRAM_TIMES(
+        "Net.QuicHpackDecoder.IndexedEntryAge",
+        base::TimeDelta::FromMicroseconds(elapsed.ToMicroseconds()));
+  }
+};
+
 }  // namespace
 
 QuicChromiumClientSession::StreamRequest::StreamRequest() : stream_(nullptr) {}
@@ -366,6 +382,14 @@ 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(