Pepper: CallOnMainThread should do nothing if Globals are gone

ppapi/proxy/ppb_core_proxy.cc

Index: ppapi/proxy/ppb_core_proxy.cc
diff --git a/ppapi/proxy/ppb_core_proxy.cc b/ppapi/proxy/ppb_core_proxy.cc
index 4c0b372fe94b3b1415469444bc7c84ff248a9558..0cf2a95ed29ed382871e171b00c5ed83edc646f8 100644
--- a/ppapi/proxy/ppb_core_proxy.cc
+++ b/ppapi/proxy/ppb_core_proxy.cc
@@ -63,6 +63,13 @@ void CallOnMainThread(int delay_in_ms,
   if (!callback.func)
     return;
   ProxyAutoLock lock;
+
+  // If the plugin attempts to call CallOnMainThread from a background thread
+  // at shutdown, it's possible that the PpapiGlobals object or the main loop
+  // has been destroyed.
+  if (!PpapiGlobals::Get() || !PpapiGlobals::Get()->GetMainThreadMessageLoop())
+    return;
+
   PpapiGlobals::Get()->GetMainThreadMessageLoop()->PostDelayedTask(
       FROM_HERE,
       RunWhileLocked(base::Bind(&CallbackWrapper, callback, result)),