Trivial followup for bug 461199: fix new compiler warning about unsigned-vs-signed comparison. (carrying forward rs=sdwilsh from previous related followup)

toolkit/components/places/tests/cpp/places_test_harness_tail.h

@@ -55,7 +55,7 @@ public:
   NS_IMETHOD Run()
   {
     NS_ASSERTION(NS_IsMainThread(), "Not running on the main thread?");
-    if (gTestsIndex < NS_ARRAY_LENGTH(gTests)) {
+    if (gTestsIndex < int(NS_ARRAY_LENGTH(gTests))) {
       do_test_pending();
       Test &test = gTests[gTestsIndex++];
       (void)fprintf(stderr, TEST_INFO_STR "Running %s.\n", TEST_FILE,