Improve memory estimate of SpdySessionPool in net/ MemoryDumpProvider.

net/http2/hpack/decoder/http2_hpack_decoder.cc

Index: net/http2/hpack/decoder/http2_hpack_decoder.cc
diff --git a/net/http2/hpack/decoder/http2_hpack_decoder.cc b/net/http2/hpack/decoder/http2_hpack_decoder.cc
index 390d8287c73d2977d19bdedfef6f00b73f35b5f0..607bec1ecba50d36c755063a4915291d6568399e 100644
--- a/net/http2/hpack/decoder/http2_hpack_decoder.cc
+++ b/net/http2/hpack/decoder/http2_hpack_decoder.cc
@@ -5,6 +5,7 @@
 #include "net/http2/hpack/decoder/http2_hpack_decoder.h"
 
 #include "base/logging.h"
+#include "base/trace_event/memory_usage_estimator.h"
 #include "net/http2/decoder/decode_status.h"
 
 using base::StringPiece;
@@ -116,6 +117,10 @@ bool Http2HpackDecoder::error_detected() {
   return error_detected_;
 }
 
+size_t Http2HpackDecoder::EstimateMemoryUsage() const {
+  return base::trace_event::EstimateMemoryUsage(entry_buffer_);
+}
+
 void Http2HpackDecoder::ReportError(StringPiece error_message) {
   DVLOG(3) << "Http2HpackDecoder::ReportError is new="
            << (!error_detected_ ? "true" : "false")