bind-gen: Apply ToV8Traits to IDL callback function

Applies ToV8Traits, a new system to convert a blink value to
a V8 value, to IDL callback function and callback interface.

Also fixes audio-encoder.any.js.
EncodedAudioChunkOutputCallback takes decoder_config as optional.
New ToV8 supports IDLOptional. If the argument for a callback function
is "missing", ToV8 returns undefined, not null.

Bug: 1172074
Change-Id: Ic63a8e4ed89f9d25b86254b1017f10374a19a8d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2711748
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Commit-Queue: Canon Mukai <canonmukai@google.com>
Cr-Commit-Position: refs/heads/master@{#857026}

webcodecs/audio-encoder.any.js

@@ -199,10 +199,10 @@ promise_test(async t => {
       // a |config| in it.
       output_count++;
       if (output_count == 1) {
-        assert_not_equals(config, null);
+        assert_equals(typeof config, "object");
         decoder_config = config;
       } else {
-        assert_equals(config, null);
+        assert_equals(config, undefined);
       }
     }
   };