Server push cancellation: add PushPromiseHelper which reflects information on the push promise.

net/quic/core/quic_client_promised_info.h

Index: net/quic/core/quic_client_promised_info.h
diff --git a/net/quic/core/quic_client_promised_info.h b/net/quic/core/quic_client_promised_info.h
index 449812c429246ccc5f0ad681039d3e091ecc59de..1ae274e53e29d933de4e7ef67aaadbd28035267f 100644
--- a/net/quic/core/quic_client_promised_info.h
+++ b/net/quic/core/quic_client_promised_info.h
@@ -78,6 +78,9 @@ class NET_EXPORT_PRIVATE QuicClientPromisedInfo
 
   const std::string url() const { return url_; }
 
+  // Return true if there's a request pending matching this push promise.
+  bool is_validating() const { return client_request_delegate_ != nullptr; }
+
  private:
   friend class test::QuicClientPromisedInfoPeer;