Comment: displayable buffer => displayable frame

"Displayable frame" is the term used in libgav1 to refer to an AV1 video
frame that can be output and is in the selected operating point.
"Displayable buffer" is less clear.

Change-Id: I1cfe176d6595e0cea37a2abf7f11e017d85e7eb5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2302738Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Wan-Teh Chang <wtc@google.com>
Cr-Commit-Position: refs/heads/master@{#789153}

media/filters/gav1_video_decoder.cc

@@ -414,7 +414,7 @@ bool Gav1VideoDecoder::MaybeDequeueFrames() {
     }
 
     if (!buffer) {
-      // This is not an error case; no displayable buffer exists or is ready.
+      // This is not an error case; no displayable frame exists or is ready.
       break;
     }