Adds the concept of Policies to the 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
index 9524ec37a34d76596985ea4f2c438e84bb52dbef..ead89bf3286a1c662b6b980ad84f09991cf30bad 100644
--- a/Source/platform/scheduler/Scheduler.h
+++ b/Source/platform/scheduler/Scheduler.h
@@ -43,8 +43,8 @@ public:
     void postTask(const TraceLocation&, const Task&); // For generic (low priority) tasks.
     void postIdleTask(const TraceLocation&, const IdleTask&); // For non-critical tasks which may be reordered relative to other task types.
 
-    // 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.
+    // Returns true if we're in low latency mode and 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 any thread.
     bool shouldYieldForHighPriorityWork() const;
 
@@ -60,6 +60,11 @@ private:
     friend class MainThreadPendingTaskRunner;
     friend class MainThreadPendingHighPriorityTaskRunner;
 
+    enum LatencyMode {
+        Normal,
+        LowLatency,

[eseidel, 2014/09/10 16:09:01]

What does LowLatency mode mean?  Maybe just a comment to explain?

+    };
+
     class TracedTask {
     public:
         TracedTask(const Task& task, const TraceLocation& location)
@@ -82,6 +87,11 @@ private:
     void tickSharedTimer();
 
     bool hasPendingHighPriorityWork() const;
+
+    // Only does work in low latency mode. Returns true if any work was done.
+    bool runPendingHighPrioirtyTasksIfInLowLatencyMode();
+
+    // Returns true if any work was done.
     bool swapQueuesAndRunPendingTasks();
     void swapQueuesRunPendingTasksAndAllowHighPriorityTaskRunnerPosting();
 
@@ -96,6 +106,12 @@ private:
     // Should only be accessed from the main thread.
     void (*m_sharedTimerFunction)();
 
+    // Should only be accessed from the main thread.
+    LatencyMode latencyMode() const;
+
+    // Can be called from any thread.
+    void setLatencyMode(LatencyMode);
+
     // These members can be accessed from any thread.
     WebThread* m_mainThread;
 
@@ -105,6 +121,12 @@ private:
 
     volatile int m_highPriorityTaskCount;
     bool m_highPriorityTaskRunnerPosted;
+
+    volatile int m_latencyMode;
+    double m_lowLatencyModeEndTimeSeconds;
+
+    // The time we should stay in low latency mode for after a touch event.
+    static double s_lowLatencyModeAfterTouchTimeSeconds;
 };
 
 } // namespace blink