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..ff847600ef810cf332b1764fe5231665fdb9f9e2 100644
--- a/net/http2/hpack/decoder/http2_hpack_decoder.cc
+++ b/net/http2/hpack/decoder/http2_hpack_decoder.cc
@@ -116,6 +116,10 @@ bool Http2HpackDecoder::error_detected() {
   return error_detected_;
 }
 
+size_t Http2HpackDecoder::EstimateMemoryUsage() const {
+  return entry_buffer_.EstimateMemoryUsage();
+}
+
 void Http2HpackDecoder::ReportError(StringPiece error_message) {
   DVLOG(3) << "Http2HpackDecoder::ReportError is new="
            << (!error_detected_ ? "true" : "false")