Add helper function for SPDY 4

net/spdy/spdy_framer.h

Index: net/spdy/spdy_framer.h
diff --git a/net/spdy/spdy_framer.h b/net/spdy/spdy_framer.h
index 9cf6b784d142950a60e87e49ecc1d4543ee695f3..f4c8abe3a330f6a0d2311f218a11f95dfd4adabd 100644
--- a/net/spdy/spdy_framer.h
+++ b/net/spdy/spdy_framer.h
@@ -488,6 +488,9 @@ class NET_EXPORT_PRIVATE SpdyFramer {
   size_t GetWindowUpdateSize() const;
   size_t GetCredentialMinimumSize() const;
 
+  // Returns the maximum payload size of a DATA frame.
+  size_t GetDataFrameMaximumPayload() const;
+
   // For debugging.
   static const char* StateToString(int state);
   static const char* ErrorCodeToString(int error_code);