views: use correct color for enabled checkbox/radio images

The existing code used FocusedBorderColor, which happened to be equal to
ProminentButtonColor on non-Mac and close enough on Mac that nobody noticed the
disparity. Mojave changed the color that underlies FocusedBorderColor, which
made checkboxes start drawing very faintly.

Bug: 918673
Change-Id: If73a4607bb5857e4b61d67cfd7e9f58ebd341021
Reviewed-on: https://chromium-review.googlesource.com/c/1394434Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619724}

ui/views/controls/button/checkbox.cc

@@ -162,7 +162,7 @@ SkColor Checkbox::GetIconImageColor(int icon_state) const {
   const SkColor active_color =
       (icon_state & IconState::CHECKED)
           ? GetNativeTheme()->GetSystemColor(
-                ui::NativeTheme::kColorId_FocusedBorderColor)
+                ui::NativeTheme::kColorId_ProminentButtonColor)
           // When unchecked, the icon color matches push button text color.
           : style::GetColor(*this, style::CONTEXT_BUTTON_MD,
                             style::STYLE_PRIMARY);