PPAPI: Make CompletionCallbacks work right on background threads.

ppapi/proxy/ppb_testing_proxy.cc

Index: ppapi/proxy/ppb_testing_proxy.cc
diff --git a/ppapi/proxy/ppb_testing_proxy.cc b/ppapi/proxy/ppb_testing_proxy.cc
index b8c9ce07d3afb86a866fe26d4ccd89448197d25b..576302f97b97071ba8f78547f4b75b1e27ed4907 100644
--- a/ppapi/proxy/ppb_testing_proxy.cc
+++ b/ppapi/proxy/ppb_testing_proxy.cc
@@ -53,12 +53,14 @@ PP_Bool ReadImageData(PP_Resource graphics_2d,
 
 void RunMessageLoop(PP_Instance instance) {
   MessageLoop::ScopedNestableTaskAllower allow(MessageLoop::current());
-  // TODO(dmichael): We should probably assert that this is the main thread.
+  CHECK(PpapiGlobals::Get()->GetMainThreadMessageLoop()->
+            BelongsToCurrentThread());
   MessageLoop::current()->Run();
 }
 
 void QuitMessageLoop(PP_Instance instance) {
-  // TODO(dmichael): We should probably assert that this is the main thread.
+  CHECK(PpapiGlobals::Get()->GetMainThreadMessageLoop()->
+            BelongsToCurrentThread());
   MessageLoop::current()->QuitNow();
 }