Bug 1780015 - Add targeting for users who prefer reduced motion in AboutWelcome message surfaces r=pdahiya

Differential Revision: https://phabricator.services.mozilla.com/D152174

browser/components/newtab/content-src/asrouter/docs/targeting-attributes.md

@@ -55,6 +55,7 @@ Please note that some targeting attributes require stricter controls on the tele
 * [doesAppNeedPrivatePin](#doesappneedprivatepin)
 * [isBackgroundTaskMode](#isbackgroundtaskmode)
 * [backgroundTaskName](#backgroundtaskname)
+* [userPrefersReducedMotion](#userPrefersReducedMotion)
 
 ## Detailed usage
 
@@ -843,3 +844,8 @@ Checks if this invocation is running in background task mode.
 
 A non-empty string task name if this invocation is running in background task
 mode, or `null` if this invocation is not running in background task mode.
+
+### `userPrefersReducedMotion`
+
+Checks if user prefers reduced motion as indicated by the value of a media query for `prefers-reduced-motion`.
+

browser/components/newtab/lib/ASRouterTargeting.jsm

@@ -741,6 +741,11 @@ const TargetingGetters = {
     );
     return bts?.backgroundTaskName();
   },
+
+  get userPrefersReducedMotion() {
+    let window = lazy.BrowserWindowTracker.getTopWindow();
+    return window?.matchMedia("(prefers-reduced-motion: reduce)")?.matches;
+  },
 };
 
 const ASRouterTargeting = {

browser/components/newtab/test/browser/browser_asrouter_targeting.js

@@ -1163,3 +1163,11 @@ add_task(async function check_isBackgroundTaskMode() {
     "Has no background task name"
   );
 });
+
+add_task(async function check_userPrefersReducedMotion() {
+  is(
+    typeof (await ASRouterTargeting.Environment.userPrefersReducedMotion),
+    "boolean",
+    "Should return a boolean"
+  );
+});