[scheduler] Prioritise input events on the compositor thread

Bind input-related mojo interface to the input task runner and add a
feature to assign the highest priority to it.

BUG=857099
[email protected],[email protected],[email protected],[email protected]
CC=​[email protected],[email protected]

Change-Id: I19f3e08bcc6af626dba298f31570102ddaffc4ac
Reviewed-on: https://chromium-review.googlesource.com/1117188
Reviewed-by: Alex Clarke <[email protected]>
Reviewed-by: Robert Kaplow <[email protected]>
Reviewed-by: Kentaro Hara <[email protected]>
Reviewed-by: Yuta Kitamura <[email protected]>
Commit-Queue: Alexander Timin <[email protected]>
Cr-Commit-Position: refs/heads/master@{#571916}
diff --git a/content/renderer/render_widget.cc b/content/renderer/render_widget.cc
index 14f771a1..247ebb4b 100644
--- a/content/renderer/render_widget.cc
+++ b/content/renderer/render_widget.cc
@@ -574,10 +574,13 @@
 
   RenderThreadImpl* render_thread_impl = RenderThreadImpl::current();
 
+  blink::scheduler::WebThreadScheduler* compositor_thread_scheduler =
+      blink::scheduler::WebThreadScheduler::CompositorThreadScheduler();
+
   widget_input_handler_manager_ = WidgetInputHandlerManager::Create(
       weak_ptr_factory_.GetWeakPtr(),
-      render_thread_impl && compositor_
-          ? render_thread_impl->compositor_task_runner()
+      compositor_thread_scheduler && compositor_
+          ? compositor_thread_scheduler->InputTaskRunner()
           : nullptr,
       render_thread_impl ? render_thread_impl->GetWebMainThreadScheduler()
                          : nullptr);