Fix some warnings on gcc 4.7 on Linux.

google_apis/gcm/engine/connection_handler_unittest.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "google_apis/gcm/engine/connection_handler.h"
 
 #include "base/bind.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/run_loop.h"
 #include "base/strings/string_number_conversions.h"
@@ skipping 33 matching lines @@
 
 // ---- Helpers for building messages. ----
 
 // Encode a protobuf packet with protobuf type |tag| and serialized protobuf
 // bytes |proto| into the MCS message form (tag + varint size + bytes).
 std::string EncodePacket(uint8 tag, const std::string& proto) {
   std::string result;
   google::protobuf::io::StringOutputStream string_output_stream(&result);
   google::protobuf::io::CodedOutputStream coded_output_stream(
       &string_output_stream);
-  const char tag_byte[1] = {tag};
+  const unsigned char tag_byte[1] = {tag};
   coded_output_stream.WriteRaw(tag_byte, 1);
   coded_output_stream.WriteVarint32(proto.size());
   coded_output_stream.WriteRaw(proto.c_str(), proto.size());
   return result;
 }
 
 // Encode a handshake request into the MCS message form.
 std::string EncodeHandshakeRequest() {
   std::string result;
   const char version_byte[1] = {kMCSVersion};
@@ skipping 552 matching lines @@
   data_message.set_category(kDataMsgCategory);
   connection_handler()->SendMessage(data_message);
   EXPECT_FALSE(connection_handler()->CanSendMessage());
   WaitForMessage();  // The message send. Should result in an error
   EXPECT_FALSE(connection_handler()->CanSendMessage());
   EXPECT_EQ(net::ERR_CONNECTION_CLOSED, last_error());
 }
 
 }  // namespace
 }  // namespace gcm