Make track.stats nullable for better feature detection.

Motivation: w3c/mediacapture-extensions#122

Will merge this CL only if the spec issue gets resolved.

Bug: chromium:1472978
Change-Id: Ie5164b12a0cce03f2ad86205a7576c798ab651e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4977420
Reviewed-by: Florent Castelli <[email protected]>
Commit-Queue: Henrik Boström <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1215524}

mediacapture-extensions/MediaStreamTrack-video-stats.https.html

@@ -326,17 +326,17 @@
   const [track] = stream.getTracks();
   t.add_cleanup(() => track.stop());
 
-  assert_throws_dom('NotSupportedError', () => track.stats);
-}, `track.stats throws on audio tracks`);
+  assert_equals(track.stats, null);
+}, `track.stats is null on audio tracks`);
 
 promise_test(async t => {
   const canvas = document.createElement('canvas');
   const stream = canvas.captureStream(10);
   const [track] = stream.getTracks();
   t.add_cleanup(() => track.stop());
 
-  assert_throws_dom('NotSupportedError', () => track.stats);
-}, `track.stats throws on non-device tracks, such as canvas`);
+  assert_equals(track.stats, null);
+}, `track.stats is null on non-device tracks, such as canvas`);
 
 promise_test(async t => {
   // getDisplayMedia() requires inducing a user gesture.