Don't route mouse wheel events to transparent children (Legacy_RenderWidgetHostHWND).

ui/base/win/mouse_wheel_util.cc

Index: ui/base/win/mouse_wheel_util.cc
diff --git a/ui/base/win/mouse_wheel_util.cc b/ui/base/win/mouse_wheel_util.cc
index 223e3a8ee5002d48d4b9d398b4e18380b197171b..020c70c17bf7c3e7ffedb3052b288909f2437ed9 100644
--- a/ui/base/win/mouse_wheel_util.cc
+++ b/ui/base/win/mouse_wheel_util.cc
@@ -101,6 +101,14 @@ bool RerouteMouseWheel(HWND window, WPARAM w_param, LPARAM l_param) {
       }
     }
 
+    // If the child window is transparent, then it is not interested in
+    // receiving wheel events.
+    if (IsChild(window, window_under_wheel) &&
+        ::GetWindowLong(
+            window_under_wheel, GWL_EXSTYLE) & WS_EX_TRANSPARENT) {
+      return false;
+    }
+
     // window_under_wheel is a Chrome window.  If allowed, redirect.
     if (IsCompatibleWithMouseWheelRedirection(window_under_wheel)) {
       base::AutoReset<bool> auto_reset_recursion_break(&recursion_break, true);