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

device/hid/hid_connection_unittest.cc

Index: device/hid/hid_connection_unittest.cc
diff --git a/device/hid/hid_connection_unittest.cc b/device/hid/hid_connection_unittest.cc
index 5e48ef9bc5ba11e95ab94bb7ca8dfc83f41471c2..3607bf237e037905e3ae47b034327cde8a245629 100644
--- a/device/hid/hid_connection_unittest.cc
+++ b/device/hid/hid_connection_unittest.cc
@@ -162,7 +162,7 @@ TEST_F(HidConnectionTest, ReadWrite) {
   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;
     }
 
@@ -175,7 +175,7 @@ TEST_F(HidConnectionTest, ReadWrite) {
     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]);
     }
   }