[webcodecs] Fix AudioDecoder |description| handling.

Previously AudioDecoder would read the whole buffer, rather than the
provided view of it.

Bug: 1094084
Change-Id: Ibdf601e1d01114456093230c5daa137807566f6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2367641Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Commit-Queue: Chrome Cunningham <chcunningham@chromium.org>
Commit-Queue: Dan Sanders <sandersd@chromium.org>
Auto-Submit: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800289}

third_party/blink/renderer/modules/webcodecs/audio_decoder.cc

@@ -58,18 +58,19 @@ CodecConfigEval AudioDecoderTraits::CreateMediaConfig(
 
   std::vector<uint8_t> extra_data;
   if (config.hasDescription()) {
-    DOMArrayBuffer* buffer;
     if (config.description().IsArrayBuffer()) {
-      buffer = config.description().GetAsArrayBuffer();
+      DOMArrayBuffer* buffer = config.description().GetAsArrayBuffer();
+      uint8_t* start = static_cast<uint8_t*>(buffer->Data());
+      size_t size = buffer->ByteLengthAsSizeT();
+      extra_data.assign(start, start + size);
     } else {
-      // TODO(sandersd): Can IsNull() be true?
       DCHECK(config.description().IsArrayBufferView());
-      buffer = config.description().GetAsArrayBufferView()->buffer();
+      DOMArrayBufferView* view =
+          config.description().GetAsArrayBufferView().Get();
+      uint8_t* start = static_cast<uint8_t*>(view->BaseAddress());
+      size_t size = view->byteLengthAsSizeT();
+      extra_data.assign(start, start + size);
     }
-    // TODO(sandersd): Is it possible to not have Data()?
-    uint8_t* start = static_cast<uint8_t*>(buffer->Data());
-    size_t size = buffer->ByteLengthAsSizeT();
-    extra_data.assign(start, start + size);
   }
 
   media::ChannelLayout channel_layout =