Add the ColorSchemeNativeThemeObserver on Mac

If certain flags are enabled, NativeThemeMac may return a different
NativeTheme instance for NativeTheme::GetInstanceForWeb(). This will
cause NativeTheme::GetInstanceForWeb() on Mac to be out of sync
in terms of dark mode and high contrast.

To fix this, NativeTheme::GetInstanceForWeb() is added as an
observer of NativeTheme::GetInstanceForNativeUi() in these cases.

Bug: 970285
Change-Id: I832d182647f81491a761a071bc5633c2efdeedb0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1834028Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Alison Maher <almaher@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#702902}

ui/native_theme/native_theme_mac.h

@@ -85,6 +85,11 @@ class NATIVE_THEME_EXPORT NativeThemeMac : public NativeThemeBase {
       appearance_observer_;
   id high_contrast_notification_token_;
 
+  // Used to notify the web native theme of changes to dark mode and high
+  // contrast.
+  std::unique_ptr<NativeTheme::ColorSchemeNativeThemeObserver>
+      color_scheme_observer_;
+
   DISALLOW_COPY_AND_ASSIGN(NativeThemeMac);
 };
 

ui/native_theme/native_theme_mac.mm

@@ -299,6 +299,15 @@ NativeThemeMac::NativeThemeMac() {
                       theme->NotifyObservers();
                     }];
   }
+
+  // Add the web native theme as an observer to stay in sync with dark mode,
+  // high contrast, and preferred color scheme changes.
+  if (features::IsFormControlsRefreshEnabled()) {
+    color_scheme_observer_ =
+        std::make_unique<NativeTheme::ColorSchemeNativeThemeObserver>(
+            NativeTheme::GetInstanceForWeb());
+    AddObserver(color_scheme_observer_.get());
+  }
 }
 
 NativeThemeMac::~NativeThemeMac() {