Bug 1332588 - Set NS_FRAME_MAY_BE_TRANSFORMED bit in nsIFrame::SetMayHaveTransformAnimation. r=boris

Without this change stacking-context-transform-changing-keyframe.html will fail
with the next change, since we will incorrectly remove
nsChangeHint_UpdateTransformLayer from the target nsIFrame if the frame doesn't
have NS_FRAME_MAY_BE_TRANSFORMED bit.

Differential Revision: https://phabricator.services.mozilla.com/D75049

layout/generic/nsIFrame.h

@@ -4415,7 +4415,10 @@ class nsIFrame : public nsQueryFrame {
   }
 
   bool MayHaveTransformAnimation() const { return mMayHaveTransformAnimation; }
-  void SetMayHaveTransformAnimation() { mMayHaveTransformAnimation = true; }
+  void SetMayHaveTransformAnimation() {
+    AddStateBits(NS_FRAME_MAY_BE_TRANSFORMED);
+    mMayHaveTransformAnimation = true;
+  }
   bool MayHaveOpacityAnimation() const { return mMayHaveOpacityAnimation; }
   void SetMayHaveOpacityAnimation() { mMayHaveOpacityAnimation = true; }