Replace NPObject usage in ppapi with gin

ppapi/tests/test_post_message.cc

Index: ppapi/tests/test_post_message.cc
diff --git a/ppapi/tests/test_post_message.cc b/ppapi/tests/test_post_message.cc
index 5413b54f00b759f5fe9ed7889bcd80cffa0c6807..6a415bbd58e9e5e906995ced5c0c743b5e7b827a 100644
--- a/ppapi/tests/test_post_message.cc
+++ b/ppapi/tests/test_post_message.cc
@@ -172,6 +172,14 @@ TestPostMessage::~TestPostMessage() {
 bool TestPostMessage::Init() {
   bool success = CheckTestingInterface();
 
+  // Add a post condition to tests which caches the postMessage function and
+  // then calls it after the instance is destroyed. The ensures that no UAF
+  // occurs because the MessageChannel may still be alive after the plugin
+  // instance is destroyed (it will get garbage collected eventually).
+  instance_->EvalScript("window.pluginPostMessage = "
+                        "document.getElementById('plugin').postMessage");
+  instance_->AddPostCondition("window.pluginPostMessage('') === undefined");
+
   // Set up a special listener that only responds to a FINISHED_WAITING string.
   // This is for use by WaitForMessages.
   std::string js_code;