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 a96618448240d4dfcd26243c3df183270fad2131..e0d651336474b248d7e9e28a86cc8fba14322dee 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn
@@ -105,6 +105,10 @@ cc_component("cc") {
     "layers/nine_patch_layer_impl.cc",
     "layers/nine_patch_layer_impl.h",
     "layers/paint_properties.h",
+    "layers/painted_overlay_scrollbar_layer.cc",
+    "layers/painted_overlay_scrollbar_layer.h",
+    "layers/painted_overlay_scrollbar_layer_impl.cc",
+    "layers/painted_overlay_scrollbar_layer_impl.h",
     "layers/painted_scrollbar_layer.cc",
     "layers/painted_scrollbar_layer.h",
     "layers/painted_scrollbar_layer_impl.cc",
@@ -282,6 +286,8 @@ 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",
@@ -807,6 +813,7 @@ cc_test("cc_unittests") {
     "playback/recording_source_unittest.cc",
     "quads/draw_polygon_unittest.cc",
     "quads/draw_quad_unittest.cc",
+    "quads/nine_patch_generator_unittest.cc",
     "quads/render_pass_unittest.cc",
     "raster/raster_buffer_provider_unittest.cc",
     "raster/scoped_gpu_raster_unittest.cc",