cc: Implement states for Task for stricter control.

cc/raster/task.h

Index: cc/raster/task.h
diff --git a/cc/raster/task.h b/cc/raster/task.h
index 96959c1ba00b0e4bf8a0e3044e698ee84bfafd8e..54bcf8b6b1a508b9d31e50fa39e0a327f144513c 100644
--- a/cc/raster/task.h
+++ b/cc/raster/task.h
@@ -13,6 +13,41 @@
 #include "cc/base/cc_export.h"
 
 namespace cc {
+class Task;
+
+// States to manage life cycle of a task. Task gets created with NEW state and
+// concludes either in FINISHED or CANCELLED state. So possible life cycle
+// paths for task are -
+//   NEW -> SCHEDULED -> RUNNING -> FINISHED
+//   NEW -> SCHEDULED -> CANCELED
+class CC_EXPORT TaskState {
+ public:
+  bool IsScheduled() const;
+  bool IsRunning() const;
+  bool IsFinished() const;
+  bool IsCanceled() const;
+
+  // Functions to change the state of task. These functions should be called
+  // only from TaskGraphWorkQueue where the life cycle of a task is decided or
+  // from tests. These functions are not thread-safe. Caller is responsible for
+  // thread safety.
+  void Reset();  // Sets state to NEW.
+  void DidSchedule();
+  void DidStart();
+  void DidFinish();
+  void DidCancel();
+
+ private:
+  friend class Task;
+
+  // Let only Task class create the TaskState.
+  TaskState();
+  ~TaskState();
+
+  enum class Value : uint16_t { NEW, SCHEDULED, RUNNING, FINISHED, CANCELED };
+
+  Value value_;
+};
 
 // A task which can be run by a TaskGraphRunner. To run a Task, it should be
 // inserted into a TaskGraph, which can then be scheduled on the
@@ -21,23 +56,21 @@ class CC_EXPORT Task : public base::RefCountedThreadSafe<Task> {
  public:
   typedef std::vector<scoped_refptr<Task>> Vector;
 
+  TaskState& state() { return state_; }
+
   // Subclasses should implement this method. RunOnWorkerThread may be called
   // on any thread, and subclasses are responsible for locking and thread
   // safety.
   virtual void RunOnWorkerThread() = 0;
 
-  void WillRun();
-  void DidRun();
-  bool HasFinishedRunning() const;
-
  protected:
   friend class base::RefCountedThreadSafe<Task>;
 
   Task();
   virtual ~Task();
 
-  bool will_run_;
-  bool did_run_;
+ private:
+  TaskState state_;
 };
 
 // A task dependency graph describes the order in which to execute a set