Make tasks cancellable inside the blink scheduler.

third_party/WebKit/Source/platform/scheduler/base/time_domain.cc

Index: third_party/WebKit/Source/platform/scheduler/base/time_domain.cc
diff --git a/third_party/WebKit/Source/platform/scheduler/base/time_domain.cc b/third_party/WebKit/Source/platform/scheduler/base/time_domain.cc
index 539fca26613f800cfae26a7ae72c6d1d62ae273e..0bdaeb3a1887ce3d233f23d5de7a09ce9a97a019 100644
--- a/third_party/WebKit/Source/platform/scheduler/base/time_domain.cc
+++ b/third_party/WebKit/Source/platform/scheduler/base/time_domain.cc
@@ -84,6 +84,37 @@ void TimeDomain::ScheduleDelayedWork(internal::TaskQueueImpl* queue,
     observer_->OnTimeDomainHasDelayedWork();
 }
 
+void TimeDomain::CancelDelayedWork(internal::TaskQueueImpl* queue,
+                                   base::TimeTicks delayed_run_time) {
+  DCHECK(main_thread_checker_.CalledOnValidThread());
+
+  auto iterpair = delayed_wakeup_multimap_.equal_range(delayed_run_time);
+  for (auto it = iterpair.first; it != iterpair.second; ++it) {
+    if (it->second == queue) {
+      base::TimeTicks prev_first_wakeup =
+          delayed_wakeup_multimap_.begin()->first;
+      delayed_wakeup_multimap_.erase(it);
+
+      if (delayed_wakeup_multimap_.empty())
+        break;
+
+      base::TimeTicks first_wakeup = delayed_wakeup_multimap_.begin()->first;
+
+      if (first_wakeup == prev_first_wakeup)

[haraken, 2016/08/19 10:42:51]

I'm just curious but what is this check doing?

[alex clarke (OOO till 29th), 2016/08/19 11:02:59]

I added a comment, it's checking if the first entry in
|delayed_wakeup_multimap_| still has the same |delayed_run_time|.  If it hasn't
changed we don't need to reschedule the wakeup.

+        break;
+
+      // The first wakeup has changed, we need to re-schedule.
+      base::TimeTicks now = Now();
+      base::TimeDelta delay = std::max(base::TimeDelta(), first_wakeup - now);
+      RequestWakeup(now, delay);
+      break;

[haraken, 2016/08/19 10:42:51]

This for loop finishes when it encounters the first task that is associated with
|queue|. Why is it enough to remove only the task encountered first?

[alex clarke (OOO till 29th), 2016/08/19 11:02:59]

I added a comment above but there might be other queues that happen to require a
wake up at |delayed_run_time| and we must respect that or timers won't fire at
the expected time.

+    }
+  }
+
+  // Note since the base TaskRunner does not support cancellation there's no
+  // point trying to cancel a wakeup requested via RequestWakeup :(
+}
+
 void TimeDomain::RegisterAsUpdatableTaskQueue(internal::TaskQueueImpl* queue) {
   {
     base::AutoLock lock(newly_updatable_lock_);