[gcc 4.x] Fix the build failure due to -Werror=char-subscripts

device/hid/hid_connection_unittest.cc

 // Copyright (c) 2014 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 <string>
 #include <vector>
 
 #include "base/bind.h"
 #include "base/callback.h"
 #include "base/memory/scoped_ptr.h"
@@ skipping 144 matching lines @@
 
   TestConnectCallback connect_callback;
   service_->Connect(device_id_, connect_callback.callback());
   scoped_refptr<HidConnection> conn = connect_callback.WaitForConnection();
   ASSERT_TRUE(conn.get());
 
   const char kBufferSize = 9;
   for (char i = 0; i < 8; ++i) {
     scoped_refptr<IOBufferWithSize> buffer(new IOBufferWithSize(kBufferSize));
     buffer->data()[0] = 0;
-    for (char j = 1; j < kBufferSize; ++j) {
+    for (unsigned char j = 1; j < kBufferSize; ++j) {
       buffer->data()[j] = i + j - 1;
     }
 
     TestIoCallback write_callback;
     conn->Write(buffer, buffer->size(), write_callback.write_callback());
     ASSERT_TRUE(write_callback.WaitForResult());
 
     TestIoCallback read_callback;
     conn->Read(read_callback.read_callback());
     ASSERT_TRUE(read_callback.WaitForResult());
     ASSERT_EQ(9UL, read_callback.size());
     ASSERT_EQ(0, read_callback.buffer()->data()[0]);
-    for (char j = 1; j < kBufferSize; ++j) {
+    for (unsigned char j = 1; j < kBufferSize; ++j) {
       ASSERT_EQ(i + j - 1, read_callback.buffer()->data()[j]);
     }
   }
 
   conn->Close();
 }
 
 }  // namespace device