Introduce a basic Blink Scheduler

Source/platform/scheduler/Scheduler.h

Index: Source/platform/scheduler/Scheduler.h
diff --git a/Source/platform/scheduler/Scheduler.h b/Source/platform/scheduler/Scheduler.h
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+++ b/Source/platform/scheduler/Scheduler.h
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+// Copyright 2014 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 Scheduler_h
+#define Scheduler_h
+
+#include "platform/PlatformExport.h"
+#include "wtf/Functional.h"
+#include "wtf/Noncopyable.h"
+
+namespace blink {
+class WebThread;
+}
+
+namespace WebCore {
+
+// The scheduler is an opinionated gateway for arranging work to be run the
+// main thread. It decides which tasks get priority over others based on a
+// scheduling policy and the overall system state.
+class PLATFORM_EXPORT Scheduler {
+    WTF_MAKE_NONCOPYABLE(Scheduler);
+public:
+    typedef Function<void()> Task;
+
+    static Scheduler* shared();
+    static void initializeOnMainThread();
+    static void shutdown();
+
+    // The following entrypoints are used to schedule different types of tasks
+    // to be run on the main thread. They can be called from any thread.
+    void postInputTask(const Task&);
+    void postCompositorTask(const Task&);
+    void postTask(const Task&); // For generic (low priority) tasks.
+
+    // Returns true if there is high priority work pending on the main thread
+    // and the caller should yield to let the scheduler service that work.
+    // Can be called on the main thread.
+    bool shouldYieldForHighPriorityWork();
+
+    // The shared timer can be used to schedule a periodic callback which may
+    // get preempted by higher priority work.
+    void setSharedTimerFiredFunction(void (*function)());
+    void setSharedTimerFireInterval(double);
+    void stopSharedTimer();
+
+private:
+    Scheduler();
+    ~Scheduler();
+
+    void scheduleTask(const Task&);
+
+    static void sharedTimerAdapter();
+    void tickSharedTimer();
+
+    static Scheduler* s_sharedScheduler;
+    blink::WebThread* m_mainThread;
+
+    void (*m_sharedTimerFunction)();
+};
+
+} // namespace WebCore
+
+#endif // Scheduler_h