Change name of NativeTheme::instance() to make it clear it's web only.

ui/native_theme/native_theme.cc

Index: ui/native_theme/native_theme.cc
diff --git a/ui/native_theme/native_theme.cc b/ui/native_theme/native_theme.cc
index 94a1eeff98c54256ac86e70ea410159e60e52c99..f15f144b8c08746c03a6f56855e65a64b5d73c61 100644
--- a/ui/native_theme/native_theme.cc
+++ b/ui/native_theme/native_theme.cc
@@ -16,9 +16,6 @@ void NativeTheme::SetScrollbarColors(unsigned inactive_color,
   track_color_ = track_color;
 }
 
-// NativeTheme::instance() is implemented in the platform specific source files,
-// such as native_theme_win.cc or native_theme_linux.cc
-
 void NativeTheme::AddObserver(NativeThemeObserver* observer) {
   native_theme_observers_.AddObserver(observer);
 }