Improve comment ImageDecoder::InitializeNewFrame

Also remove unneeded ImageFrame setter calls in
AVIFImageDecoder::InitializeNewFrame. By analyzing all the overrides of
the InitializeNewFrame method, I found that the following ImageFrame
setters can only be called during InitializeNewFrame:
  * SetDuration
  * SetDisposalMethod
  * SetAlphaBlendSource
  * SetRequiredPreviousFrameIndex
Since AVIF frames don't require any previous frames, we don't need to
call the last three setters in AVIFImageDecoder::InitializeNewFrame.

Change-Id: I554c1a28a4dc008cd5e19946297b82fa0061f97c
Bug: 1071221
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2164108
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783760}

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

@@ -450,23 +450,13 @@ void AVIFImageDecoder::InitializeNewFrame(size_t index) {
   if (decode_to_half_float_)
     buffer.SetPixelFormat(ImageFrame::PixelFormat::kRGBA_F16);
 
+  // For AVIFs, the frame always fills the entire image.
   buffer.SetOriginalFrameRect(IntRect(IntPoint(), Size()));
 
   avifImageTiming timing;
   auto ret = avifDecoderNthImageTiming(decoder_.get(), index, &timing);
   DCHECK_EQ(ret, AVIF_RESULT_OK);
   buffer.SetDuration(base::TimeDelta::FromSecondsD(timing.duration));
-
-  // The AVIF file format does not contain information equivalent to the
-  // disposal method or alpha blend source. Since the AVIF decoder handles frame
-  // dependence internally, set options that best correspond to "each frame is
-  // independent".
-  buffer.SetDisposalMethod(ImageFrame::kDisposeNotSpecified);
-  buffer.SetAlphaBlendSource(ImageFrame::kBlendAtopBgcolor);
-
-  // Leave all frames as being independent (the default) because we require all
-  // frames be the same size.
-  DCHECK_EQ(buffer.RequiredPreviousFrameIndex(), kNotFound);
 }
 
 void AVIFImageDecoder::Decode(size_t index) {

third_party/blink/renderer/platform/image-decoders/image_decoder.h

@@ -444,8 +444,8 @@ class PLATFORM_EXPORT ImageDecoder {
   // call.
   bool InitFrameBuffer(size_t);
 
-  // Performs any additional setup of the requested frame after it has been
-  // initially created, e.g. setting a duration or disposal method.
+  // Performs any decoder-specific setup of the requested frame after it has
+  // been newly created, e.g. setting the frame's duration or disposal method.
   virtual void InitializeNewFrame(size_t) {}
 
   // Decodes the requested frame.