Fix DCHECK in destroying MainThreadEventQueue.

The OneShotTimer needs to be destroyed on the main thread. Destroy it
during ClearClient.

BUG=1112931

Change-Id: I9fb7946947943f7ff2a5bbb19d7ae1af94335b08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2347392
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796850}

third_party/blink/renderer/platform/widget/input/main_thread_event_queue.cc

@@ -246,9 +246,9 @@ MainThreadEventQueue::MainThreadEventQueue(
       needs_unbuffered_input_for_debugger_(false),
       allow_raf_aligned_input_(allow_raf_aligned_input),
       main_task_runner_(main_task_runner),
-      main_thread_scheduler_(main_thread_scheduler),
-      use_raf_fallback_timer_(true) {
-  raf_fallback_timer_.SetTaskRunner(main_task_runner);
+      main_thread_scheduler_(main_thread_scheduler) {
+  raf_fallback_timer_ = std::make_unique<base::OneShotTimer>();
+  raf_fallback_timer_->SetTaskRunner(main_task_runner);
 
   event_predictor_ = std::make_unique<InputEventPrediction>(
       base::FeatureList::IsEnabled(blink::features::kResamplingInputEvents));
@@ -508,8 +508,13 @@ void MainThreadEventQueue::RafFallbackTimerFired() {
   DispatchRafAlignedInput(base::TimeTicks::Now());
 }
 
+void MainThreadEventQueue::ClearRafFallbackTimerForTesting() {
+  raf_fallback_timer_.reset();
+}
+
 void MainThreadEventQueue::DispatchRafAlignedInput(base::TimeTicks frame_time) {
-  raf_fallback_timer_.Stop();
+  if (raf_fallback_timer_)
+    raf_fallback_timer_->Stop();
   size_t queue_size_at_start;
 
   // Record the queue size so that we only process
@@ -676,8 +681,8 @@ bool MainThreadEventQueue::HandleEventOnMainThread(
 
 void MainThreadEventQueue::SetNeedsMainFrame() {
   if (main_task_runner_->BelongsToCurrentThread()) {
-    if (use_raf_fallback_timer_) {
-      raf_fallback_timer_.Start(
+    if (raf_fallback_timer_) {
+      raf_fallback_timer_->Start(
           FROM_HERE, kMaxRafDelay,
           base::BindOnce(&MainThreadEventQueue::RafFallbackTimerFired, this));
     }
@@ -696,6 +701,7 @@ void MainThreadEventQueue::SetNeedsMainFrame() {
 void MainThreadEventQueue::ClearClient() {
   DCHECK(main_task_runner_->BelongsToCurrentThread());
   client_ = nullptr;
+  raf_fallback_timer_.reset();
 }
 
 void MainThreadEventQueue::SetNeedsLowLatency(bool low_latency) {

third_party/blink/renderer/platform/widget/input/main_thread_event_queue.h

@@ -141,9 +141,7 @@ class PLATFORM_EXPORT MainThreadEventQueue
       const std::unique_ptr<MainThreadEventQueueTask>& item) const;
   void RafFallbackTimerFired();
 
-  void set_use_raf_fallback_timer(bool use_timer) {
-    use_raf_fallback_timer_ = use_timer;
-  }
+  void ClearRafFallbackTimerForTesting();
 
   friend class QueuedWebInputEvent;
   friend class MainThreadEventQueueTest;
@@ -175,8 +173,10 @@ class PLATFORM_EXPORT MainThreadEventQueue
 
   scoped_refptr<base::SingleThreadTaskRunner> main_task_runner_;
   scheduler::WebThreadScheduler* main_thread_scheduler_;
-  base::OneShotTimer raf_fallback_timer_;
-  bool use_raf_fallback_timer_;
+
+  // A safe guard timer to ensure input is always processed. A BeginMainFrame
+  // signal might not always occur if our visibility changed.
+  std::unique_ptr<base::OneShotTimer> raf_fallback_timer_;
 
   std::unique_ptr<InputEventPrediction> event_predictor_;
 

third_party/blink/renderer/platform/widget/input/main_thread_event_queue_unittest.cc

@@ -185,7 +185,7 @@ class MainThreadEventQueueTest : public testing::Test,
   void SetUp() override {
     queue_ = new MainThreadEventQueue(this, main_task_runner_,
                                       &thread_scheduler_, true);
-    queue_->set_use_raf_fallback_timer(false);
+    queue_->ClearRafFallbackTimerForTesting();
   }
 
   void HandleEvent(WebInputEvent& event,