Call PostDecodeProcessing() at the end of Decode()

Call PostDecodeProcessing() when Decode() sets the status of an image
frame buffer to ImageFrame::kFrameComplete. This matches the Decode()
methods of the other image decoders that support animation
(GIFImageDecoder, PNGImageDecoder, and WEBPImageDecoder). This is
related to the UpdateAggressivePurging() call at the beginning of
Decode().

Change-Id: Ib98db567cdb492fcbcdaad36d055e73177f651df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2536112Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827108}

third_party/blink/renderer/platform/image-decoders/avif/avif_image_decoder.cc

@@ -575,6 +575,7 @@ void AVIFImageDecoder::Decode(size_t index) {
   buffer.SetPixelsChanged(true);
   buffer.SetHasAlpha(!!image->alphaPlane);
   buffer.SetStatus(ImageFrame::kFrameComplete);
+  PostDecodeProcessing(index);
 }
 
 bool AVIFImageDecoder::CanReusePreviousFrameBuffer(size_t index) const {