[webcodecs] [MSE]: Keep Encoded{Audio,Video}Chunks non-dictionary

Drops a couple obsolete TODOs which previously tracked changing
EncodedAudioChunk and EncodedVideoChunk to IDL dictionary types.
This doesn't preclude adding parallel dictionary types that the APIs
can consume later, if needed.

For the MSE case, keeping them as non-IDL dictionaries retains the
current ability for the bindings generator to precisely disambiguate
them (for example in matching to an overload or union). This CL
intends to increase certainty that that disambiguation capability
will remain; otherwise new API shapes depending on it might need to
change.

Note that keeping them as non-dictionary types is nice for webcodecs;
it better retains their immutability and enables potential for
reduced/zero-copy scenarios (sandersd@ mentioned this, along with
mentioning improved implementation control over behavior like
construction and transferring).

BUG=1144908

Change-Id: Ib9fb6bcd50b6fe2900908c3380b75b798c71b667
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2535735Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827392}

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

@@ -32,8 +32,6 @@
   // Request decoding of an input chunk.
   //
   // You must call configure() before calling decode() for the first time.
-  //
-  // TODO(chcunningham): Change to a dictionary type.
   [RaisesException] void decode(EncodedAudioChunk chunk);
 
   // Request output from all previous decode requests.

third_party/blink/renderer/modules/webcodecs/video_decoder.idl

@@ -44,8 +44,6 @@
   // Request decoding of an input chunk.
   //
   // You must call configure() before calling decode() for the first time.
-  //
-  // TODO(sandersd): Change to a dictionary type.
   [RaisesException] void decode(EncodedVideoChunk chunk);
 
   // Request output from all previous decode requests.