Remove Windows dependency in NativeThemeBase::GetControlColor()

With CL:2587618, we no longer need to check for a Windows dependency
in NativeThemeBase::GetControlColor(). Instead, we can call
InForcedColorsMode(), which in practice is only ever true on
Windows.

There is work underway to add a devtools emulation feature for Windows
high contrast to allow developers to test their webpages in forced
colors mode without a Windows device. This change will likely allow
for controls to be rendered correctly in this case.

This CL should not result in any functional changes.

Bug: 1107431
Change-Id: I268e73ea574dd55c535ae3bb031c1d06b84d1d48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2590589
Commit-Queue: Alison Maher <almaher@microsoft.com>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Ionel Popescu <iopopesc@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#840200}

ui/native_theme/native_theme_base.cc

@@ -1607,10 +1607,8 @@ SkColor NativeThemeBase::ControlsBackgroundColorForState(
 
 SkColor NativeThemeBase::GetControlColor(ControlColorId color_id,
                                          ColorScheme color_scheme) const {
-#if defined(OS_WIN)
-  if (UserHasContrastPreference() && features::IsForcedColorsEnabled())
+  if (InForcedColorsMode() && features::IsForcedColorsEnabled())
     return GetHighContrastControlColor(color_id, color_scheme);
-#endif
 
   if(color_scheme == ColorScheme::kDark)
     return GetDarkModeControlColor(color_id);