Fix GCC4.7 compilation warning introduced in r10400

test/cctest/test-api.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 13707 matching lines @@
 TEST(VisitExternalStrings) {
   v8::HandleScope scope;
   LocalContext env;
   const char* string = "Some string";
   uint16_t* two_byte_string = AsciiToTwoByteString(string);
   TestResource* resource1 = new TestResource(two_byte_string);
   v8::Local<v8::String> string1 = v8::String::NewExternal(resource1);
   TestResource* resource2 = new TestResource(two_byte_string);
   v8::Local<v8::String> string2 = v8::String::NewExternal(resource2);
 
+  // We need to add usages for string1 and string2 to avoid warnings in GCC 4.7
+  CHECK(string1->IsExternal());
+  CHECK(string2->IsExternal());
+
   VisitorImpl visitor(resource1, resource2);
   v8::V8::VisitExternalResources(&visitor);
   visitor.CheckVisitedResources();
 }
 
 
 static double DoubleFromBits(uint64_t value) {
   double target;
   memcpy(&target, &value, sizeof(target));
   return target;
@@ skipping 2142 matching lines @@
   CompileRun("throw 'exception';");
 }
 
 
 TEST(CallCompletedCallbackTwoExceptions) {
   v8::HandleScope scope;
   LocalContext env;
   v8::V8::AddCallCompletedCallback(CallCompletedCallbackException);
   CompileRun("throw 'first exception';");
 }