Issue #2861 - Part 2 Follow-up: nsIPluginHost.isPluginOOP should not always return true.

Actually restore the method as it was when it was added in Mozilla bug 1178896.

dom/plugins/base/nsPluginHost.cpp

@@ -1063,7 +1063,11 @@ NS_IMETHODIMP
 nsPluginHost::IsPluginOOP(const nsACString& aMimeType,
                           bool* aResult)
 {
-  *aResult = true;
+  nsPluginTag* tag = FindNativePluginForType(aMimeType, true);
+  if (!tag) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+  *aResult = nsNPAPIPlugin::RunPluginOOP(tag);
   return NS_OK;
 }