Bug 1853712 - Allow blob image recording for nsDisplayButtonBorder. r=nical

These only paint inner box shadows and borders, both of which we support with
WR display items, so we probably never hit fallback rendering for this and don't
call this method. But in case we do, it should be fine to use blob rendering
for it these days.

Depends on D188490

Differential Revision: https://phabricator.services.mozilla.com/D188491

layout/forms/nsButtonFrameRenderer.cpp

@@ -340,8 +340,6 @@ class nsDisplayButtonBorder final : public nsPaintedDisplayItem {
   }
   MOZ_COUNTED_DTOR_OVERRIDE(nsDisplayButtonBorder)
 
-  virtual bool MustPaintOnContentSide() const override { return true; }
-
   virtual void HitTest(nsDisplayListBuilder* aBuilder, const nsRect& aRect,
                        HitTestState* aState,
                        nsTArray<nsIFrame*>* aOutFrames) override {