Explain why FrameIsReceivedAtIndex() is correct

Bug: 1147863
Change-Id: Ia3186b0c09d3b73a14e0745ffb0a3b2fb12b1a3d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533915Reviewed-by: Urvang Joshi <urvang@chromium.org>
Reviewed-by: James Zern <jzern@google.com>
Commit-Queue: Wan-Teh Chang <wtc@google.com>
Cr-Commit-Position: refs/heads/master@{#827063}

third_party/blink/renderer/platform/image-decoders/webp/webp_image_decoder.cc

@@ -279,6 +279,11 @@ bool WEBPImageDecoder::FrameIsReceivedAtIndex(size_t index) const {
     return false;
   if (!(format_flags_ & ANIMATION_FLAG))
     return ImageDecoder::FrameIsReceivedAtIndex(index);
+  // frame_buffer_cache_.size() is equal to the return value of
+  // DecodeFrameCount(). WebPDemuxGetI(demux_, WEBP_FF_FRAME_COUNT) returns the
+  // number of ANMF chunks that have been received. (See also the DCHECK on
+  // animated_frame.complete in InitializeNewFrame().) Therefore we can return
+  // true if |index| is valid for frame_buffer_cache_.
   bool frame_is_received_at_index = index < frame_buffer_cache_.size();
   return frame_is_received_at_index;
 }