Replace the existing SpdyHeaderBlock typedef with a class.

net/spdy/spdy_framer.cc

Index: net/spdy/spdy_framer.cc
diff --git a/net/spdy/spdy_framer.cc b/net/spdy/spdy_framer.cc
index a12c3ceada035a7328bad4343eece9ec68c3fd32..ad9fb975284efe9afbbeff1e2420308d5e7336e2 100644
--- a/net/spdy/spdy_framer.cc
+++ b/net/spdy/spdy_framer.cc
@@ -1330,7 +1330,7 @@ void SpdyFramer::WriteHeaderBlockToZ(const SpdyHeaderBlock* headers,
 
   WriteLengthZ(headers->size(), length_length, kZStandardData, z);
 
-  std::map<std::string, std::string>::const_iterator it;
+  SpdyHeaderBlock::const_iterator it;
   for (it = headers->begin(); it != headers->end(); ++it) {
     WriteLengthZ(it->first.size(), length_length, kZStandardData, z);
     WriteZ(it->first, kZStandardData, z);