Fix GCC4.7 compilation warning introduced in r10400

test/cctest/test-api.cc

Index: test/cctest/test-api.cc
===================================================================
--- test/cctest/test-api.cc	(revision 10403)
+++ test/cctest/test-api.cc	(working copy)
@@ -13725,6 +13725,10 @@
   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();