Use dark mode colors for incognito buttons

This was broken by the GM2 style update. Since these constants are only used in the infobar, we can unconditionally use dark mode style.

Bug: 940900
Change-Id: I9da25d945da94e72c4e0c7d8a08dfd8ca021196c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1539125
Auto-Submit: Leonard Grey <lgrey@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644777}

ui/native_theme/native_theme_dark_aura.cc

@@ -26,11 +26,11 @@ SkColor NativeThemeDarkAura::GetSystemColor(ColorId color_id) const {
 
     // Button
     case kColorId_ButtonEnabledColor:
-      return SK_ColorWHITE;
+      return gfx::kGoogleGrey200;
     case kColorId_ProminentButtonFocusedColor:
-      return gfx::kGoogleBlue500;
+      return gfx::kGoogleBlue400;
     case kColorId_ProminentButtonColor:
-      return gfx::kGoogleBlue600;
+      return gfx::kGoogleBlue300;
 
     // Label
     case kColorId_LabelEnabledColor: