scheduler: Add new platform interface for implementing scheduling

public/platform/WebScheduler.h

Index: public/platform/WebScheduler.h
diff --git a/public/platform/WebScheduler.h b/public/platform/WebScheduler.h
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+++ b/public/platform/WebScheduler.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 WebScheduler_h
+#define WebScheduler_h
+
+namespace blink {
+
+class WebTraceLocation;
+
+// This class is used to submit tasks and pass other information from Blink to
+// the platform's scheduler.
+class WebScheduler {
+public:
+    virtual ~WebScheduler() { }
+
+    // An IdleTask is passed a deadline in CLOCK_MONOTONIC seconds and is
+    // expected to complete before this deadline.
+    class IdleTask {
+    public:
+        virtual ~IdleTask() { }
+        virtual void run(double deadlineSeconds) = 0;
+    };
+
+    // Called during Blink's shutdown to delete any pending tasks from the
+    // scheduler. Must be called on the main thread.
+    virtual void shutdown() { }
+
+    // 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.
+    // Must be called on the main thread.
+    virtual bool shouldYieldForHighPriorityWork() { return false; }
+
+    // Schedule an idle task to run the Blink main thread. For non-critical
+    // tasks which may be reordered relative to other task types and may be
+    // starved for an arbitrarily long time if no idle time is available.
+    // Takes ownership of |IdleTask|. Can be called from any thread.
+    virtual void postIdleTask(const WebTraceLocation&, IdleTask*) { }
+};
+
+} // namespace blink
+
+#endif // WebScheduler_h