Add NULL check for NativeTheme in scrollbar methods. A NULL theme can

ui/views/controls/scrollbar/native_scroll_bar_views.cc

Index: ui/views/controls/scrollbar/native_scroll_bar_views.cc
diff --git a/ui/views/controls/scrollbar/native_scroll_bar_views.cc b/ui/views/controls/scrollbar/native_scroll_bar_views.cc
index 06e96a2e23d0510019b58deaf1e3e04373430eda..f9ac51ca64e79e068fe024ec4541156336c0e72f 100644
--- a/ui/views/controls/scrollbar/native_scroll_bar_views.cc
+++ b/ui/views/controls/scrollbar/native_scroll_bar_views.cc
@@ -382,6 +382,8 @@ NativeScrollBarWrapper* NativeScrollBarWrapper::CreateWrapper(
 // static
 int NativeScrollBarWrapper::GetHorizontalScrollBarHeight(
     const ui::NativeTheme* theme) {
+  if (!theme)
+    theme = ui::NativeTheme::instance();
   ui::NativeTheme::ExtraParams button_params;
   button_params.scrollbar_arrow.is_hovering = false;
   gfx::Size button_size = theme->GetPartSize(
@@ -402,6 +404,8 @@ int NativeScrollBarWrapper::GetHorizontalScrollBarHeight(
 // static
 int NativeScrollBarWrapper::GetVerticalScrollBarWidth(
     const ui::NativeTheme* theme) {
+  if (!theme)
+    theme = ui::NativeTheme::instance();
   ui::NativeTheme::ExtraParams button_params;
   button_params.scrollbar_arrow.is_hovering = false;
   gfx::Size button_size = theme->GetPartSize(