[Browser Settings] Add comments to clarify MultideviceHandler usage.

This CL adds a comment in settings_ui.cc to clarify usage of
MultideivceHandler.

Bug: 845706
Change-Id: Ib2d1b34dac11b52ba8374f146400f3a9c5ac8001
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2006618
Commit-Queue: Azeem Arshad <azeemarshad@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732720}

chrome/browser/ui/webui/settings/settings_ui.cc

@@ -315,6 +315,9 @@ void SettingsUI::InitBrowserSettingsWebUIHandlers() {
             account_manager, IdentityManagerFactory::GetForProfile(profile)));
   }
 
+  // MultideviceHandler is required in browser settings to show a special note
+  // under the notification permission that is auto-granted for Android Messages
+  // integration in ChromeOS.
   if (!profile->IsGuestSession()) {
     chromeos::android_sms::AndroidSmsService* android_sms_service =
         chromeos::android_sms::AndroidSmsServiceFactory::GetForBrowserContext(