[debugger] only track LiveEdit if source changes.

R=hablich@chromium.org

Review URL: https://codereview.chromium.org/1486003003

Cr-Commit-Position: refs/heads/master@{#32450}

src/runtime/runtime-liveedit.cc

@@ -229,9 +229,14 @@ RUNTIME_FUNCTION(Runtime_LiveEditCompareStrings) {
   CONVERT_ARG_HANDLE_CHECKED(String, s1, 0);
   CONVERT_ARG_HANDLE_CHECKED(String, s2, 1);
 
+  Handle<JSArray> result = LiveEdit::CompareStrings(s1, s2);
+  uint32_t array_length;
+  CHECK(result->length()->ToArrayLength(&array_length));
+  if (array_length > 0) {
     isolate->debug()->feature_tracker()->Track(DebugFeatureTracker::kLiveEdit);
+  }
 
-  return *LiveEdit::CompareStrings(s1, s2);
+  return *result;
 }