Revert "Fix crash with MojoInputMessages"

This reverts commit 14aa2e0e.

Reason for revert:
<select> tags non-functional (both web and chrome:// pages).
See http://crbug.com/776548

Original change's description:
> 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 since it is no longer valid.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/725860
> Reviewed-by: Mustaq Ahmed <mustaq@chromium.org>
> Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#509894}

TBR=dtapuska@chromium.org,mustaq@chromium.org

Change-Id: I7e8f0d3f6a8e41acf12bda09993aac1b2cbfccf3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 775483
Reviewed-on: https://chromium-review.googlesource.com/729206Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510232}

content/renderer/input/widget_input_handler_manager.cc

@@ -132,9 +132,7 @@ void WidgetInputHandlerManager::AddInterface(
   }
 }
 
-void WidgetInputHandlerManager::WillShutdown() {
-  input_handler_proxy_.reset();
-}
+void WidgetInputHandlerManager::WillShutdown() {}
 
 void WidgetInputHandlerManager::TransferActiveWheelFlingAnimation(
     const blink::WebActiveWheelFlingParameters& params) {
@@ -245,14 +243,7 @@ void WidgetInputHandlerManager::ObserveGestureEventOnMainThread(
 void WidgetInputHandlerManager::DispatchEvent(
     std::unique_ptr<content::InputEvent> event,
     mojom::WidgetInputHandler::DispatchEventCallback callback) {
-  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);
-    }
+  if (!event || !event->web_event) {
     return;
   }
 
@@ -381,7 +372,7 @@ void WidgetInputHandlerManager::HandledInputEvent(
                                   touch_action));
   } else {
     std::move(callback).Run(
-        InputEventAckSource::MAIN_THREAD, latency_info, ack_state,
+        InputEventAckSource::COMPOSITOR_THREAD, latency_info, ack_state,
         overscroll_params
             ? base::Optional<ui::DidOverscrollParams>(*overscroll_params)
             : base::nullopt,
@@ -392,8 +383,6 @@ 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);