Add additional tracing for mojo input

Add some additional trace methods for when mojo input is received.

Bug: None
Change-Id: I84e78ffaac21995894f20f6778e83ba1213c8d06
Reviewed-on: https://chromium-review.googlesource.com/998672Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549218}

content/browser/renderer_host/input/input_router_impl.cc

@@ -432,6 +432,8 @@ void InputRouterImpl::FilterAndSendWebInputEvent(
   InputEventAckState filtered_state =
       client_->FilterInputEvent(input_event, latency_info);
   if (WasHandled(filtered_state)) {
+    TRACE_EVENT_INSTANT0("input", "InputEventFiltered",
+                         TRACE_EVENT_SCOPE_THREAD);
     if (filtered_state != INPUT_EVENT_ACK_STATE_UNKNOWN) {
       std::move(callback).Run(InputEventAckSource::BROWSER, latency_info,
                               filtered_state, base::nullopt, base::nullopt);
@@ -443,10 +445,14 @@ void InputRouterImpl::FilterAndSendWebInputEvent(
       ScaleEvent(input_event, device_scale_factor_), latency_info);
   if (WebInputEventTraits::ShouldBlockEventStream(
           input_event, wheel_scroll_latching_enabled_)) {
+    TRACE_EVENT_INSTANT0("input", "InputEventSentBlocking",
+                         TRACE_EVENT_SCOPE_THREAD);
     client_->IncrementInFlightEventCount();
     client_->GetWidgetInputHandler()->DispatchEvent(std::move(event),
                                                     std::move(callback));
   } else {
+    TRACE_EVENT_INSTANT0("input", "InputEventSentNonBlocking",
+                         TRACE_EVENT_SCOPE_THREAD);
     client_->GetWidgetInputHandler()->DispatchNonBlockingEvent(
         std::move(event));
     std::move(callback).Run(InputEventAckSource::BROWSER, latency_info,

content/renderer/input/widget_input_handler_impl.cc

@@ -131,15 +131,15 @@ void WidgetInputHandlerImpl::RequestCompositionUpdates(bool immediate_request,
 void WidgetInputHandlerImpl::DispatchEvent(
     std::unique_ptr<content::InputEvent> event,
     DispatchEventCallback callback) {
-  if (!event || !event->web_event) {
-    return;
-  }
+  TRACE_EVENT0("input", "WidgetInputHandlerImpl::DispatchEvent");
   input_handler_manager_->DispatchEvent(std::move(event), std::move(callback));
 }
 
 void WidgetInputHandlerImpl::DispatchNonBlockingEvent(
     std::unique_ptr<content::InputEvent> event) {
-  DispatchEvent(std::move(event), DispatchEventCallback());
+  TRACE_EVENT0("input", "WidgetInputHandlerImpl::DispatchNonBlockingEvent");
+  input_handler_manager_->DispatchEvent(std::move(event),
+                                        DispatchEventCallback());
 }
 
 void WidgetInputHandlerImpl::AttachSynchronousCompositor(