More aggressively yield when awaiting touchstart response

Change the RendererScheduler's |ShouldYieldForHighPriorityWork| query
to always return true if we are awaiting touchstart response, regardless
of whether any high-priority work has been queued. This should facilitate
more aggressive yielding immediately after touchstart, reducing the risk
of long-running tasks interrupting dispatch of the subsequent touchmove.

BUG=452956

Review URL: https://codereview.chromium.org/933293004

Cr-Commit-Position: refs/heads/master@{#317130}

content/renderer/scheduler/renderer_scheduler_impl.cc

@@ -190,14 +190,25 @@ bool RendererSchedulerImpl::ShouldYieldForHighPriorityWork() {
     return false;
 
   MaybeUpdatePolicy();
-  // We only yield if we are in the touchstart/compositor priority and there
-  // is input/compositor work outstanding. Note: even though the control queue
-  // is higher priority we don't yield for it since these tasks are not
-  // user-provided work and they are only intended to run before the next task,
-  // not interrupt the tasks.
-  return (SchedulerPolicy() == COMPOSITOR_PRIORITY_POLICY ||
-          SchedulerPolicy() == TOUCHSTART_PRIORITY_POLICY) &&
-         !task_queue_manager_->IsQueueEmpty(COMPOSITOR_TASK_QUEUE);
+  // We only yield if we are in the compositor priority and there is compositor
+  // work outstanding, or if we are in the touchstart response priority.
+  // Note: even though the control queue is higher priority we don't yield for
+  // it since these tasks are not user-provided work and they are only intended
+  // to run before the next task, not interrupt the tasks.
+  switch (SchedulerPolicy()) {
+    case NORMAL_PRIORITY_POLICY:
+      return false;
+
+    case COMPOSITOR_PRIORITY_POLICY:
+      return !task_queue_manager_->IsQueueEmpty(COMPOSITOR_TASK_QUEUE);
+
+    case TOUCHSTART_PRIORITY_POLICY:
+      return true;
+
+    default:
+      NOTREACHED();
+      return false;
+  }
 }
 
 void RendererSchedulerImpl::CurrentIdleTaskDeadlineCallback(

content/renderer/scheduler/renderer_scheduler_impl_unittest.cc

@@ -798,6 +798,14 @@ TEST_F(RendererSchedulerImplTest, TestShouldYield) {
   // We should be able to switch to compositor priority mid-task.
   EXPECT_FALSE(should_yield_before);
   EXPECT_TRUE(should_yield_after);
+
+  // Receiving a touchstart should immediately trigger yielding, even if
+  // there's no immediately pending work in the compositor queue.
+  EXPECT_FALSE(scheduler_->ShouldYieldForHighPriorityWork());
+  scheduler_->DidReceiveInputEventOnCompositorThread(
+      FakeInputEvent(blink::WebInputEvent::TouchStart));
+  EXPECT_TRUE(scheduler_->ShouldYieldForHighPriorityWork());
+  RunUntilIdle();
 }
 
 }  // namespace content