Feed ScrollableArea::showOverlayScrollbars into ScrollbarAnimationController.

cc/blink/web_layer_impl.h

Index: cc/blink/web_layer_impl.h
diff --git a/cc/blink/web_layer_impl.h b/cc/blink/web_layer_impl.h
index b0ef4f6fb9db8a3b2d6e7a0c7e0baadf87aaf6fd..c54cd9a0dd7c6142cda333ab276c8c743d9fe091 100644
--- a/cc/blink/web_layer_impl.h
+++ b/cc/blink/web_layer_impl.h
@@ -126,6 +126,7 @@ class CC_BLINK_EXPORT WebLayerImpl : public NON_EXPORTED_BASE(blink::WebLayer) {
   void setCompositorMutableProperties(uint32_t properties) override;
   uint32_t compositorMutableProperties() const override;
   void setHasWillChangeTransformHint(bool has_will_change) override;
+  void showScrollbars() override;
 
   void setScrollParent(blink::WebLayer* parent) override;
   void setClipParent(blink::WebLayer* parent) override;