Fix flaky layout tests with Mojo Input Messages on.

When I landed MojoInputMessages by default yesterday in
https://chromium-review.googlesource.com/789590 the
dispatchKeyEvent-focus.js layout test became flaky.

The cause was layout tests don't use a compositor thread and events
could have been placed out of order because events hitting the
WidgetInputHandlerImpl (setFocus would get queued in the
input_event_queue_ whereas events received would get executed right away)

There is no need for the input_event_queue_ to be passed when there is
no compositor_task_queue_ as that causes an extra thread hop that isn't
necessary.

BUG=722928

Change-Id: I6d40ec67072e2ef98f2ebac04e6e8d54c7282ad0
Reviewed-on: https://chromium-review.googlesource.com/793972Reviewed-by: Mustaq Ahmed <mustaq@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519829}

content/renderer/input/widget_input_handler_manager.cc

@@ -290,8 +290,11 @@ void WidgetInputHandlerManager::BindAssociatedChannel(
     mojom::WidgetInputHandlerAssociatedRequest request) {
   if (!request.is_pending())
     return;
+  // Don't pass the |input_event_queue_| on if we don't have a
+  // |compositor_task_runner_| as events might get out of order.
   WidgetInputHandlerImpl* handler = new WidgetInputHandlerImpl(
-      this, main_thread_task_runner_, input_event_queue_, render_widget_);
+      this, main_thread_task_runner_,
+      compositor_task_runner_ ? input_event_queue_ : nullptr, render_widget_);
   handler->SetAssociatedBinding(std::move(request));
 }
 
@@ -299,8 +302,11 @@ void WidgetInputHandlerManager::BindChannel(
     mojom::WidgetInputHandlerRequest request) {
   if (!request.is_pending())
     return;
+  // Don't pass the |input_event_queue_| on if we don't have a
+  // |compositor_task_runner_| as events might get out of order.
   WidgetInputHandlerImpl* handler = new WidgetInputHandlerImpl(
-      this, main_thread_task_runner_, input_event_queue_, render_widget_);
+      this, main_thread_task_runner_,
+      compositor_task_runner_ ? input_event_queue_ : nullptr, render_widget_);
   handler->SetBinding(std::move(request));
 }