[scheduler] Extra protection against empty tasks in scheduler

R=skyostil@chromium.org
BUG=789901

Change-Id: I39e845e80772571c9bac3c0d5f53fb04c3c53be5
Reviewed-on: https://chromium-review.googlesource.com/823031
Commit-Queue: Alexander Timin <altimin@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524414}

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

@@ -410,7 +410,7 @@ TaskQueueImpl::WakeUpForDelayedWork(LazyNow* lazy_now) {
   while (!main_thread_only().delayed_incoming_queue.empty()) {
     Task& task =
         const_cast<Task&>(main_thread_only().delayed_incoming_queue.top());
-    if (task.task.IsCancelled()) {
+    if (!task.task || task.task.IsCancelled()) {
       main_thread_only().delayed_incoming_queue.pop();
       continue;
     }

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

@@ -477,7 +477,10 @@ TaskQueueManager::ProcessTaskResult TaskQueueManager::ProcessTaskFromWorkQueue(
       work_queue->TakeTaskFromWorkQueue();
 
   // It's possible the task was canceled, if so bail out.
-  if (pending_task.task.IsCancelled())
+  // The task should be non-null, but it seems to be possible to due
+  // a hard-to-track bug. The first check is a defence against this bug,
+  // and this check isn't expected to be true in practice.
+  if (!pending_task.task || pending_task.task.IsCancelled())
     return ProcessTaskResult::kExecuted;
 
   internal::TaskQueueImpl* queue = work_queue->task_queue();

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

@@ -97,7 +97,7 @@ TaskQueueImpl::Task WorkQueue::TakeTaskFromWorkQueue() {
   // Skip over canceled tasks, except for the last one since we always return
   // something.
   while (work_queue_.size() > 1u) {
-    if (work_queue_.front().task.IsCancelled()) {
+    if (!work_queue_.front().task || work_queue_.front().task.IsCancelled()) {
       work_queue_.pop_front();
     } else {
       break;