Summarizer API: Fix summarizeStreaming WPT

Check the concatenated text stream, rather than the last chunk.

Fixed: 417493972
Change-Id: Ia3fde37e011efff04a431e4ef61a3c4d2f3015ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6542867
Auto-Submit: Mike Wasserman <[email protected]>
Commit-Queue: Mike Wasserman <[email protected]>
Reviewed-by: Daseul Lee <[email protected]>
Commit-Queue: Daseul Lee <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1459650}

Author: Mike Wasserman

ai/summarizer/summarizer-summarize-streaming.tentative.https.window.js

@@ -19,7 +19,7 @@ promise_test(async t => {
     if (done) {
       break;
     }
-    result = value;
+    result += value;
   }
   assert_greater_than(result.length, 0);
 }, 'Summarizer.summarizeStreaming() returns ReadableStream with a non-empty text.');