Fix Mac build of r4860

Build error:
test/cctest/test-api.cc: In function 'void ExternalArrayInfoTestHelper(v8::ExternalArrayType)':
test/cctest/test-api.cc:10069: warning: control may reach end of non-void function 'int ExternalArrayElementSize(v8::ExternalArrayType)' being inlined

TBR=ager@chromium.org
Review URL: http://codereview.chromium.org/2826002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4861 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

test/cctest/test-api.cc

@@ -9671,6 +9671,8 @@ static int ExternalArrayElementSize(v8::ExternalArrayType array_type) {
       UNREACHABLE();
       return -1;
   }
+  UNREACHABLE();
+  return -1;
 }