Built-In AI APIs: Check instanceof in WPTs

Minor followup from crrev.com/c/6648569

Bug: None
Change-Id: I816a7c07dea743a1bba71a81f4f32d458cd97d7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7101613
Reviewed-by: Nathan Memmott <memmott@chromium.org>
Commit-Queue: Nathan Memmott <memmott@chromium.org>
Auto-Submit: Mike Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1538770}

Author: Mike Wasserman

ai/rewriter/rewriter-create-available.tentative.https.window.js

@@ -7,7 +7,7 @@
 
 promise_test(async () => {
   const rewriter = await createRewriter();
-  assert_equals(typeof rewriter, 'object');
+  assert_true(rewriter instanceof Rewriter);
 
   assert_equals(typeof rewriter.rewrite, 'function');
   assert_equals(typeof rewriter.rewriteStreaming, 'function');

ai/summarizer/summarizer-create-available.tentative.https.window.js

@@ -7,7 +7,7 @@
 
 promise_test(async () => {
   const summarizer = await createSummarizer();
-  assert_equals(typeof summarizer, 'object');
+  assert_true(summarizer instanceof Summarizer);
 
   assert_equals(typeof summarizer.summarize, 'function');
   assert_equals(typeof summarizer.summarizeStreaming, 'function');

ai/writer/writer-create-available.tentative.https.window.js

@@ -7,7 +7,7 @@
 
 promise_test(async () => {
   const writer = await createWriter();
-  assert_equals(typeof writer, 'object');
+  assert_true(writer instanceof Writer);
 
   assert_equals(typeof writer.write, 'function');
   assert_equals(typeof writer.writeStreaming, 'function');