Add DCHECK(!RuntimeEnabledFeatures::BlinkGenPropertyTreesEnabled()) in cc::Layer::CurrentScrollOffset(). The scroll offset from the cc::Layer should not be used in BGPT mode.

https://cs.chromium.org/chromium/src/cc/layers/layer.h?rcl=a3d09957b77601d50dfdbc6b79d649ff421022eb&l=388