feat: Add will-resize event to TopLevelWindow (#13649)

The new `will-resize` event can be used to prevent the resize from
happening before the native window is actually resized. This is in
contrast to the existing the `resize` event, which is sent *after* the
native window has already been resized. For apps with e.g. custom window
snapping logic, the `resize` event is not sufficient because it will
result in flickering between the dragged size and the snapped size.

`will-resize` is only emitted on macOS and Windows.

atom/browser/native_window.h

@@ -236,6 +236,8 @@ class NativeWindow : public base::SupportsUserData,
   void NotifyWindowMinimize();
   void NotifyWindowRestore();
   void NotifyWindowMove();
+  void NotifyWindowWillResize(const gfx::Rect& new_bounds,
+                              bool* prevent_default);
   void NotifyWindowResize();
   void NotifyWindowMoved();
   void NotifyWindowScrollTouchBegin();