Use nine-patch resource for drawing Aura overlay scrollbar thumb.

cc/BUILD.gn

Index: cc/BUILD.gn
diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 8a535036e8ef208df132fa1b757b143a3b17db89..10cdf76a27e1b5f00ac11a9229c5b01e4defae4c 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn
@@ -126,6 +126,10 @@ cc_component("cc") {
     "layers/nine_patch_layer.h",
     "layers/nine_patch_layer_impl.cc",
     "layers/nine_patch_layer_impl.h",
+    "layers/overlay_scrollbar_layer.cc",
+    "layers/overlay_scrollbar_layer.h",
+    "layers/overlay_scrollbar_layer_impl.cc",
+    "layers/overlay_scrollbar_layer_impl.h",
     "layers/paint_properties.h",
     "layers/painted_scrollbar_layer.cc",
     "layers/painted_scrollbar_layer.h",
@@ -314,6 +318,8 @@ cc_component("cc") {
     "quads/draw_quad.h",
     "quads/largest_draw_quad.cc",
     "quads/largest_draw_quad.h",
+    "quads/nine_patch_generator.cc",
+    "quads/nine_patch_generator.h",
     "quads/picture_draw_quad.cc",
     "quads/picture_draw_quad.h",
     "quads/render_pass.cc",