workers: Introduce a WorkerMessageFilter to ease routing/processing worker messages.

content/child/worker_thread_message_filter.h

Index: content/child/worker_thread_message_filter.h
diff --git a/content/child/worker_thread_message_filter.h b/content/child/worker_thread_message_filter.h
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+// Copyright 2015 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 CONTENT_CHILD_WORKER_THREAD_MESSAGE_FILTER_H_
+#define CONTENT_CHILD_WORKER_THREAD_MESSAGE_FILTER_H_
+
+#include "content/child/child_message_filter.h"
+
+namespace base {
+class SingleThreadTaskRunner;
+}
+
+namespace content {
+
+class ThreadSafeSender;
+
+// A base class for filtering IPC messages targeted for worker threads.
+class WorkerThreadMessageFilter : public ChildMessageFilter {
+ public:
+  explicit WorkerThreadMessageFilter(ThreadSafeSender* thread_safe_sender);
+
+ protected:
+  ~WorkerThreadMessageFilter() override;
+
+  base::SingleThreadTaskRunner* main_thread_task_runner() {
+    return main_thread_task_runner_.get();
+  }
+  ThreadSafeSender* thread_safe_sender() { return thread_safe_sender_.get(); }
+
+ private:
+  // Returns whether this filter should process |msg|.
+  virtual bool ShouldHandleMessage(const IPC::Message& msg) const = 0;
+
+  // Processes the IPC message in the worker thread, if the filter could extract
+  // its thread id. Otherwise, runs in the main thread. It only receives a
+  // message if ShouldHandleMessage() returns true for it.
+  virtual void OnFilteredMessageReceived(const IPC::Message& msg) = 0;
+
+  // Attempts to extract the thread-id of the worker-thread that should process
+  // the IPC message. Returns whether the thread-id could be determined and set
+  // in |ipc_thread_id|.
+  virtual bool GetWorkerThreadIdForMessage(const IPC::Message& msg,
+                                           int* ipc_thread_id) = 0;
+
+  // ChildMessageFilter implementation:
+  base::TaskRunner* OverrideTaskRunnerForMessage(const IPC::Message& msg) final;
+  bool OnMessageReceived(const IPC::Message& msg) final;
+
+  scoped_refptr<base::SingleThreadTaskRunner> main_thread_task_runner_;
+  scoped_refptr<ThreadSafeSender> thread_safe_sender_;
+
+  DISALLOW_COPY_AND_ASSIGN(WorkerThreadMessageFilter);
+};
+
+}  // namespace content
+
+#endif  // CONTENT_CHILD_WORKER_THREAD_MESSAGE_FILTER_H_