Remove unnecessary DCHECK in TouchEventQueue handler notification

Previously, the TouchEventQueue would flush itself when notified that
all touch handlers have been removed. As that is no longer the case,
remove the assert verifying that the queue is empty when notified that
a touch handler has been added.

BUG=406916

Review URL: https://codereview.chromium.org/569793003

Cr-Commit-Position: refs/heads/master@{#294711}

content/browser/renderer_host/input/touch_event_queue.cc

@@ -626,7 +626,6 @@ void TouchEventQueue::OnHasTouchEventHandlers(bool has_handlers) {
     if (touch_filtering_state_ == DROP_ALL_TOUCHES) {
       // If no touch handler was previously registered, ensure that we don't
       // send a partial touch sequence to the renderer.
-      DCHECK(touch_queue_.empty());
       touch_filtering_state_ = DROP_TOUCHES_IN_SEQUENCE;
     }
   } else {

content/browser/renderer_host/input/touch_event_queue_unittest.cc

@@ -388,6 +388,15 @@ TEST_F(TouchEventQueueTest, ActiveSequenceDroppedWhenHandlersRemoved) {
   EXPECT_EQ(0U, GetAndResetAckedEventCount());
   EXPECT_EQ(2U, queued_event_count());
 
+  // Repeated registration/unregstration of handlers should have no effect as
+  // we're still awaiting the ack arrival.
+  OnHasTouchEventHandlers(true);
+  EXPECT_EQ(0U, GetAndResetAckedEventCount());
+  EXPECT_EQ(2U, queued_event_count());
+  OnHasTouchEventHandlers(false);
+  EXPECT_EQ(0U, GetAndResetAckedEventCount());
+  EXPECT_EQ(2U, queued_event_count());
+
   // The ack should be flush the queue.
   SendTouchEventAck(INPUT_EVENT_ACK_STATE_NO_CONSUMER_EXISTS);
   EXPECT_EQ(2U, GetAndResetAckedEventCount());