MacViews: Propagate theme change to widget

This is literally a copy and paste job from native_widget_aura. I'd like
to push this logic down into NativeWidgetPrivate but DesktopNativeWidgetAura
also subclasses it and seems to do its own thing, so I'm open to suggestions
on how to best deduplicate.

Bug: 1015514
Change-Id: Ib3bb2c6507190802ce540d3bcde9526ec9239bdf
Reviewed-on: https://chromium-review.googlesource.com/1015515Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551758}

ui/views/widget/native_widget_mac.mm

@@ -175,7 +175,10 @@ bool NativeWidgetMac::ShouldWindowContentsBeTransparent() const {
 }
 
 void NativeWidgetMac::FrameTypeChanged() {
-  NOTIMPLEMENTED();
+  // This is called when the Theme has changed; forward the event to the root
+  // widget.
+  GetWidget()->ThemeChanged();
+  GetWidget()->GetRootView()->SchedulePaint();
 }
 
 Widget* NativeWidgetMac::GetWidget() {