Browser plugin should never handle MouseWheels.

content/renderer/browser_plugin/browser_plugin.cc

Index: content/renderer/browser_plugin/browser_plugin.cc
diff --git a/content/renderer/browser_plugin/browser_plugin.cc b/content/renderer/browser_plugin/browser_plugin.cc
index 1380398df26a3ccbd1d165d46a1d68e813558baa..035f37795b07d2852ba31cd0e9fbd4ac2cad6747 100644
--- a/content/renderer/browser_plugin/browser_plugin.cc
+++ b/content/renderer/browser_plugin/browser_plugin.cc
@@ -448,11 +448,15 @@ blink::WebInputEventResult BrowserPlugin::handleInputEvent(
 
   DCHECK(!blink::WebInputEvent::isTouchEventType(event.type()));
 
-  if (event.type() == blink::WebInputEvent::MouseWheel) {
-    auto wheel_event = static_cast<const blink::WebMouseWheelEvent&>(event);
-    if (wheel_event.resendingPluginId == browser_plugin_instance_id_)
-      return blink::WebInputEventResult::NotHandled;
-  }
+  // With direct event routing turned on, BrowserPlugin should almost never
+  // see wheel events any more. The two exceptions are (1) scroll bubbling, and
+  // (2) synthetic mouse wheels generated by touchpad GesturePinch events on
+  // Mac, which always go to the mainframe and thus may hit BrowserPlugin if
+  // it's in a top-level embedder. In both cases we should indicate the event
+  // as not handled (for GesturePinch on Mac, indicating the event has been
+  // handled leads to touchpad pinch not working).
+  if (event.type() == blink::WebInputEvent::MouseWheel)
+    return blink::WebInputEventResult::NotHandled;
 
   if (blink::WebInputEvent::isGestureEventType(event.type())) {
     auto gesture_event = static_cast<const blink::WebGestureEvent&>(event);