PPAPI: Make CompletionCallbacks work right on background threads.

ppapi/shared_impl/tracked_callback.h

Index: ppapi/shared_impl/tracked_callback.h
diff --git a/ppapi/shared_impl/tracked_callback.h b/ppapi/shared_impl/tracked_callback.h
index 1fa7b1bce88a1bf688e61891ded618638c614864..03bf1626e30882cf09b986c8f7f61f28388cce3c 100644
--- a/ppapi/shared_impl/tracked_callback.h
+++ b/ppapi/shared_impl/tracked_callback.h
@@ -16,10 +16,12 @@
 #include "ppapi/c/pp_instance.h"
 #include "ppapi/c/pp_resource.h"
 #include "ppapi/shared_impl/ppapi_shared_export.h"
+#include "ppapi/shared_impl/ppb_message_loop_shared.h"
 
 namespace ppapi {
 
 class CallbackTracker;
+class MessageLoopShared;
 class Resource;
 
 namespace thunk {
@@ -76,7 +78,6 @@ class PPAPI_SHARED_EXPORT TrackedCallback
   // Run() will invoke the call immediately, if invoked from the target thread
   // (as determined by target_loop_). If invoked on a different thread, the
   // callback will be scheduled to run later on target_loop_.
-  // TODO(dmichael): Make the above part about different threads actually true.
   void Run(int32_t result);
   // PostRun is like Run(), except it guarantees that the callback will be run
   // later. In particular, if you invoke PostRun on the same thread on which the
@@ -116,6 +117,9 @@ class PPAPI_SHARED_EXPORT TrackedCallback
     return (callback_.func &&
             (callback_.flags & PP_COMPLETIONCALLBACK_FLAG_OPTIONAL));
   }
+  bool has_null_target_loop() const {
+    return target_loop_ == NULL;
+  }
 
  private:
   // TrackedCallback and EnterBase manage dealing with how to invoke callbacks
@@ -146,6 +150,10 @@ class PPAPI_SHARED_EXPORT TrackedCallback
   bool aborted_;
   PP_CompletionCallback callback_;
 
+  // The MessageLoopShared on which this callback should be run. This will be
+  // NULL if we're in-process.
+  scoped_refptr<MessageLoopShared> target_loop_;
+
   int32_t result_for_blocked_callback_;
   // Used for pausing/waking the blocked thread if this is a blocking completion
   // callback. Note that in-process, there is no lock, blocking callbacks are