webrtc wpt: fix wrong assert description

copy-paste probably

BUG=chromium:1124967

Change-Id: Iba5fc35c093489724fb3bf6759e8417bb212a550
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391076Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Cr-Commit-Position: refs/heads/master@{#804747}

third_party/blink/web_tests/external/wpt/webrtc/RTCRtpTransceiver-setCodecPreferences.html

@@ -110,7 +110,7 @@
         tried = true;
       }
     });
-    assert_true(tried, 'VP8 video codec was found and tried');
+    assert_true(tried, 'H264 video codec was found and tried');
   }, `setCodecPreferences() with only H264 should succeed`);
 
   async function getRTPMapLinesWithCodecAsFirst(firstCodec)