Fix crash with MojoInputMessages

Once WillShutdown is called the InputHandlerProxy object should be destroyed
since it is no longer valid.

BUG=775483

Change-Id: I3dc1cf7d7295ff08bbcec3d8ee2b12690961af2e
Reviewed-on: https://chromium-review.googlesource.com/725860Reviewed-by: Mustaq Ahmed <mustaq@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509894}

content/renderer/input/widget_input_handler_manager.cc

@@ -132,7 +132,9 @@ void WidgetInputHandlerManager::AddInterface(
   }
 }
 
-void WidgetInputHandlerManager::WillShutdown() {}
+void WidgetInputHandlerManager::WillShutdown() {
+  input_handler_proxy_.reset();
+}
 
 void WidgetInputHandlerManager::TransferActiveWheelFlingAnimation(
     const blink::WebActiveWheelFlingParameters& params) {
@@ -243,7 +245,14 @@ void WidgetInputHandlerManager::ObserveGestureEventOnMainThread(
 void WidgetInputHandlerManager::DispatchEvent(
     std::unique_ptr<content::InputEvent> event,
     mojom::WidgetInputHandler::DispatchEventCallback callback) {
-  if (!event || !event->web_event) {
+  if (!event || !event->web_event || !input_handler_proxy_) {
+    // Call |callback| if it was available indicating this event wasn't
+    // handled.
+    if (callback) {
+      std::move(callback).Run(
+          InputEventAckSource::MAIN_THREAD, ui::LatencyInfo(),
+          INPUT_EVENT_ACK_STATE_NOT_CONSUMED, base::nullopt, base::nullopt);
+    }
     return;
   }
 
@@ -372,7 +381,7 @@ void WidgetInputHandlerManager::HandledInputEvent(
                                   touch_action));
   } else {
     std::move(callback).Run(
-        InputEventAckSource::COMPOSITOR_THREAD, latency_info, ack_state,
+        InputEventAckSource::MAIN_THREAD, latency_info, ack_state,
         overscroll_params
             ? base::Optional<ui::DidOverscrollParams>(*overscroll_params)
             : base::nullopt,
@@ -383,6 +392,8 @@ void WidgetInputHandlerManager::HandledInputEvent(
 void WidgetInputHandlerManager::ObserveGestureEventOnCompositorThread(
     const blink::WebGestureEvent& gesture_event,
     const cc::InputHandlerScrollResult& scroll_result) {
+  if (!input_handler_proxy_)
+    return;
   DCHECK(input_handler_proxy_->scroll_elasticity_controller());
   input_handler_proxy_->scroll_elasticity_controller()
       ->ObserveGestureEventAndResult(gesture_event, scroll_result);