Introduce EventProcessor::OnEventProcessingStarted()

ui/aura/window_event_dispatcher.cc

Index: ui/aura/window_event_dispatcher.cc
diff --git a/ui/aura/window_event_dispatcher.cc b/ui/aura/window_event_dispatcher.cc
index 70d4e588f656c979e9f91232a86aabf0c453e14f..0b1103d016880e8015092b7a734b0a5145a34acc 100644
--- a/ui/aura/window_event_dispatcher.cc
+++ b/ui/aura/window_event_dispatcher.cc
@@ -435,16 +435,12 @@ ui::EventTarget* WindowEventDispatcher::GetRootTarget() {
   return window();
 }
 
-void WindowEventDispatcher::PrepareEventForDispatch(ui::Event* event) {
-  if (dispatching_held_event_) {
-    // The held events are already in |window()|'s coordinate system. So it is
-    // not necessary to apply the transform to convert from the host's
-    // coordinate system to |window()|'s coordinate system.
-    return;
-  }
-  if (event->IsLocatedEvent()) {
+void WindowEventDispatcher::OnEventProcessingStarted(ui::Event* event) {
+  // The held events are already in |window()|'s coordinate system. So it is
+  // not necessary to apply the transform to convert from the host's
+  // coordinate system to |window()|'s coordinate system.
+  if (event->IsLocatedEvent() && !dispatching_held_event_)
     TransformEventForDeviceScaleFactor(static_cast<ui::LocatedEvent*>(event));
-  }
 }
 
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