Make tasks cancellable inside the blink scheduler.

third_party/WebKit/public/platform/scheduler/base/task_queue.h

Index: third_party/WebKit/public/platform/scheduler/base/task_queue.h
diff --git a/third_party/WebKit/public/platform/scheduler/base/task_queue.h b/third_party/WebKit/public/platform/scheduler/base/task_queue.h
index 9dc47a325956e74bf4ccc6025fdbe84020a5b6ff..a38cb330afe30c750e592e99735054409b8660ab 100644
--- a/third_party/WebKit/public/platform/scheduler/base/task_queue.h
+++ b/third_party/WebKit/public/platform/scheduler/base/task_queue.h
@@ -8,7 +8,6 @@
 #include "base/macros.h"
 #include "base/message_loop/message_loop.h"
 #include "base/single_thread_task_runner.h"
-#include "base/trace_event/trace_event.h"
 #include "public/platform/WebCommon.h"
 
 namespace base {
@@ -19,6 +18,10 @@ class BlameContext;
 
 namespace blink {
 namespace scheduler {
+namespace internal {
+class TaskQueueImpl;
+}  // namespace internal
+class FakeWebTaskRunner;
 class LazyNow;
 class TimeDomain;
 
@@ -136,6 +139,49 @@ class BLINK_PLATFORM_EXPORT TaskQueue : public base::SingleThreadTaskRunner {
     bool should_report_when_execution_blocked;
   };
 
+  // Intended to be used as an opaque handle to a task posted by
+  // PostCancellableDelayedTask.
+  class BLINK_PLATFORM_EXPORT TaskHandle {
+   public:
+    TaskHandle();
+
+    // Returns false if the handle is equivalent to TaskHandle(), i.e. the
+    // handle doesn't represent a task that got posted.
+    operator bool() const;
+
+   private:
+    friend internal::TaskQueueImpl;
+    friend FakeWebTaskRunner;
+
+    // For immediate tasks.
+    TaskHandle(TaskQueue* task_queue, uint64_t enqueue_order);
+
+    // For delayed tasks.
+    TaskHandle(TaskQueue* task_queue,
+               base::TimeTicks scheduled_run_time,
+               int sequence_number);
+
+    uint64_t enqueue_order_;
+    base::TimeTicks scheduled_run_time_;
+#if DCHECK_IS_ON()
+    TaskQueue* task_queue_;
+#endif
+    int sequence_number_;
+  };
+
+  // Posts the given task to be run after |delay| has passed. Returns a handle
+  // which can be passed to CancelTask to cancel the task before it has run.
+  // NOTE this must be called on the thread this TaskQueue was created by.
+  virtual TaskHandle PostCancellableDelayedTask(
+      const tracked_objects::Location& from_here,
+      const base::Closure& task,
+      base::TimeDelta delay) = 0;
+
+  // Attempts to cancel a task posted by PostCancellableDelayedTask. Returns
+  // true on success or false otherwise. NOTE this must be called on the thread
+  // this TaskQueue was created by.
+  virtual bool CancelTask(const TaskHandle& handle) = 0;
+
   // Enable or disable task execution for this queue. NOTE this must be called
   // on the thread this TaskQueue was created by.
   virtual void SetQueueEnabled(bool enabled) = 0;