Adds a DebugString() method to QuicHeaderList.

net/quic/quic_header_list.cc

Index: net/quic/quic_header_list.cc
diff --git a/net/quic/quic_header_list.cc b/net/quic/quic_header_list.cc
index 2edb3438d75a52216f65f5b51c5d2559a3bff5c6..09e1dc424d29cd8b49dd2ac44264459eca3d94e9 100644
--- a/net/quic/quic_header_list.cc
+++ b/net/quic/quic_header_list.cc
@@ -37,4 +37,13 @@ void QuicHeaderList::Clear() {
   uncompressed_header_bytes_ = 0;
 }
 
+std::string QuicHeaderList::DebugString() const {
+  std::string s = "{ ";
+  for (const auto& p : *this) {
+    s.append(p.first + "=" + p.second + ", ");
+  }
+  s.append("}");
+  return s;
+}
+
 }  // namespace net