PPAPI: Make CompletionCallbacks work right on background threads.

ppapi/shared_impl/ppb_message_loop_shared.h

Index: ppapi/shared_impl/ppb_message_loop_shared.h
diff --git a/ppapi/shared_impl/ppb_message_loop_shared.h b/ppapi/shared_impl/ppb_message_loop_shared.h
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+++ b/ppapi/shared_impl/ppb_message_loop_shared.h
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+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef PPAPI_SHARED_IMPL_PPB_MESSAGE_LOOP_SHARED_H_
+#define PPAPI_SHARED_IMPL_PPB_MESSAGE_LOOP_SHARED_H_
+
+#include "base/basictypes.h"
+#include "base/callback_forward.h"
+#include "base/location.h"
+#include "ppapi/c/pp_instance.h"
+#include "ppapi/shared_impl/ppapi_shared_export.h"
+#include "ppapi/shared_impl/resource.h"
+#include "ppapi/thunk/ppb_message_loop_api.h"
+
+namespace tracked_objects {
+class Location;
+}
+
+namespace ppapi {
+
+// MessageLoopShared doesn't really do anything interesting. It exists so that
+// shared code (in particular, TrackedCallback) can keep a pointer to a
+// MessageLoop resource. In the host side, there is not a concrete class that
+// implements this. So pointers to MessageLoopShared can only really be valid
+// on the plugin side.
+class PPAPI_SHARED_EXPORT MessageLoopShared
+    : public Resource,
+      public thunk::PPB_MessageLoop_API {
+ public:
+  explicit MessageLoopShared(PP_Instance instance);
+  // Construct the one MessageLoopShared for the main thread. This must be
+  // invoked on the main thread.
+  struct ForMainThread {};
+  explicit MessageLoopShared(ForMainThread);
+  virtual ~MessageLoopShared();
+
+  // Handles posting to the message loop if there is one, or the pending queue
+  // if there isn't.
+  // NOTE: The given closure will be run *WITHOUT* acquiring the Proxy lock.
+  //       This only makes sense for user code and completely thread-safe
+  //       proxy operations (e.g., MessageLoop::QuitClosure).
+  virtual void PostClosure(const tracked_objects::Location& from_here,
+                           const base::Closure& closure,
+                           int64 delay_ms) = 0;
+
+  DISALLOW_COPY_AND_ASSIGN(MessageLoopShared);
+};
+
+}  // namespace ppapi
+
+#endif  // PPAPI_SHARED_IMPL_PPB_MESSAGE_LOOP_SHARED_H_