Disable overlay scrollbars in Blink when hidden by the compositor.

cc/proto/begin_main_frame_and_commit_state.proto

Index: cc/proto/begin_main_frame_and_commit_state.proto
diff --git a/cc/proto/begin_main_frame_and_commit_state.proto b/cc/proto/begin_main_frame_and_commit_state.proto
index c90bb6a65901c7d9c93009d3e9829575a2f5ea38..ebb94e50d85bcdac4e74f8eca2ca83b5fc4dba67 100644
--- a/cc/proto/begin_main_frame_and_commit_state.proto
+++ b/cc/proto/begin_main_frame_and_commit_state.proto
@@ -16,11 +16,17 @@ message ScrollUpdateInfo {
   optional Vector2d scroll_delta = 2;
 }
 
+message ScrollbarsUpdateInfo {
+  optional int64 layer_id = 1;
+  optional bool hidden = 2;
+}
+
 message ScrollAndScaleSet {
   repeated ScrollUpdateInfo scrolls = 1;
   optional float page_scale_delta = 2;
   optional Vector2dF elastic_overscroll_delta = 3;
   optional float top_controls_delta = 4;
+  repeated ScrollbarsUpdateInfo scrollbars = 5;
 
   // TODO(khushalsagar): Do we need to send swap promises?
   // See crbug/576999.