If there is no compositor_task_runner queue Scroll Event to main thread

In web tests there is no compositor_task_runner. In this case we need to
use the main_thread_task_runner.

Bug: 1047198
Change-Id: I23cf2f46543eff84d11e694e81b2ce2e74b96b03
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2451324Reviewed-by: David Bokan <bokan@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Liviu Tinta <liviutinta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815389}

third_party/blink/public/platform/input/input_handler_proxy.h

@@ -52,8 +52,10 @@ class MomentumScrollJankTracker;
 
 // This class is a proxy between the blink web input events for a WebWidget and
 // the compositor's input handling logic. InputHandlerProxy instances live
-// entirely on the compositor thread. Each InputHandler instance handles input
-// events intended for a specific WebWidget.
+// entirely on the compositor thread if one exists; however, it can exist on
+// the main thread in web tests where only a single thread is used.
+// Each InputHandler instance handles input events intended for a specific
+// WebWidget.
 class BLINK_PLATFORM_EXPORT InputHandlerProxy
     : public cc::InputHandlerClient,
       public SynchronousInputHandlerProxy,

third_party/blink/renderer/platform/widget/input/widget_input_handler_manager.cc

@@ -397,14 +397,16 @@ void WidgetInputHandlerManager::DispatchScrollGestureToCompositor(
   std::unique_ptr<WebCoalescedInputEvent> web_scoped_gesture_event =
       std::make_unique<WebCoalescedInputEvent>(std::move(event),
                                                ui::LatencyInfo());
-  DCHECK(compositor_task_runner_);
-  compositor_task_runner_->PostTask(
-      FROM_HERE, base::BindOnce(&WidgetInputHandlerManager::
-                                    HandleInputEventWithLatencyInfoOnCompositor,
+  // input thread task runner is |main_thread_task_runner_| only in tests
+  InputThreadTaskRunner()->PostTask(
+      FROM_HERE,
+      base::BindOnce(&WidgetInputHandlerManager::
+                         HandleInputEventWithLatencyOnInputHandlingThread,
                      this, std::move(web_scoped_gesture_event)));
 }
 
-void WidgetInputHandlerManager::HandleInputEventWithLatencyInfoOnCompositor(
+void WidgetInputHandlerManager::
+    HandleInputEventWithLatencyOnInputHandlingThread(
         std::unique_ptr<WebCoalescedInputEvent> event) {
   DCHECK(base::FeatureList::IsEnabled(features::kScrollUnification));
   DCHECK(input_handler_proxy_);

third_party/blink/renderer/platform/widget/input/widget_input_handler_manager.h

@@ -227,7 +227,7 @@ class PLATFORM_EXPORT WidgetInputHandlerManager final
       const WebGestureEvent& gesture_event,
       const cc::InputHandlerScrollResult& scroll_result);
 
-  void HandleInputEventWithLatencyInfoOnCompositor(
+  void HandleInputEventWithLatencyOnInputHandlingThread(
       std::unique_ptr<WebCoalescedInputEvent>);
 
   // Returns the task runner for the thread that receives input. i.e. the