Update return type of availableOnDevice API

This CL updates the return type of the availableOnDevice API to reflect
the spec changes in this PR: github.com/WebAudio/web-speech-api/pull/143

Bug:40286514
Change-Id: I66e4a18e5cdaf262cecd847eca6b2205618028ef

speech-api/SpeechRecognition-availableOnDevice.https.html

@@ -14,13 +14,17 @@
     "availableOnDevice should return a Promise."
   );
 
-  // Verify the resolved value is a boolean.
+  // Verify the resolved value is a string.
   const result = await resultPromise;
   assert_true(
-    typeof result === "boolean",
-    "The resolved value of the availableOnDevice promise should be a boolean."
+    typeof result === "string",
+    "The resolved value of the availableOnDevice promise should be a string."
   );
-}, "SpeechRecognition.availableOnDevice resolves with a boolean value.");
+
+  assert_true(result === "available",
+    "The resolved value of the availableOnDevice promise should be a valid value."
+  );
+}, "SpeechRecognition.availableOnDevice resolves with a string value.");
 
 promise_test(async (t) => {
   const iframe = document.createElement("iframe");