Prevent overlay scrollbars expand or hover together

cc/BUILD.gn

Index: cc/BUILD.gn
diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 7641d0b35fc2454f3ea7895ed1826f76bf0fec31..ba46e1d2aa25dbeab43b5943e54f67e3d0630d42 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn
@@ -95,6 +95,8 @@ cc_component("cc") {
     "input/scrollbar_animation_controller_thinning.cc",
     "input/scrollbar_animation_controller_thinning.h",
     "input/selection.h",
+    "input/single_scrollbar_animation_controller_thinning.cc",
+    "input/single_scrollbar_animation_controller_thinning.h",
     "layers/append_quads_data.h",
     "layers/content_layer_client.h",
     "layers/draw_properties.cc",
@@ -812,6 +814,7 @@ test("cc_unittests") {
     "input/scroll_state_unittest.cc",
     "input/scrollbar_animation_controller_linear_fade_unittest.cc",
     "input/scrollbar_animation_controller_thinning_unittest.cc",
+    "input/single_scrollbar_animation_controller_thinning_unittest.cc",
     "ipc/cc_param_traits_unittest.cc",
     "ipc/struct_traits_unittest.cc",
     "layers/heads_up_display_layer_impl_unittest.cc",