Added OverwriteFlags method to SpdyFrameBuilder.

net/spdy/spdy_frame_builder.cc

Index: net/spdy/spdy_frame_builder.cc
diff --git a/net/spdy/spdy_frame_builder.cc b/net/spdy/spdy_frame_builder.cc
index da11be0ca60dfad14c16da2759b825407955e82c..1df7d4668b44d170a6696444b9cbfe6eb1af6256 100644
--- a/net/spdy/spdy_frame_builder.cc
+++ b/net/spdy/spdy_frame_builder.cc
@@ -182,6 +182,18 @@ bool SpdyFrameBuilder::OverwriteLength(const SpdyFramer& framer,
   return success;
 }
 
+bool SpdyFrameBuilder::OverwriteFlags(const SpdyFramer& framer,
+                                      uint8 flags) {
+  DCHECK_LE(SPDY4, framer.protocol_version());
+  bool success = false;
+  const size_t old_length = length_;
+  // Flags are the fourth octet in the frame prefix.
+  length_ = 3;
+  success = WriteUInt8(flags);
+  length_ = old_length;
+  return success;
+}
+
 bool SpdyFrameBuilder::CanWrite(size_t length) const {
   if (length > kLengthMask) {
     DCHECK(false);