MSE: Make changeType-play layout tests' audio mime-type be audio-specific

If using video/webm for an audio codec/stream, some user agents will say
the type is unsupported where otherwise it would be if it were
audio/<bytestream-format> for the same codec/stream.

This change makes the types in audio test metadata match the stream types
more closely (audio/webm;... and audio/mp4;...).

This change also includes a drive-by comment addition to help ensure the
audioOnlyTypes in mediasource-util.js remain correct relative to
the changeType tests' assumptions that none of the audioOnlyTypes
generate timestamps automatically.

BUG=871929
WPT issue https://github.com/web-platform-tests/wpt/issues/12336

Change-Id: I768035b817ad46f9f17f7dc681c3f9f5c1dad8e4
Reviewed-on: https://chromium-review.googlesource.com/1166030Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581369}

third_party/WebKit/LayoutTests/external/wpt/media-source/mediasource-changetype-util.js

@@ -19,12 +19,12 @@ function findSupportedChangeTypeTestTypes(cb)
             url: 'mp4/test-v-128k-320x240-24fps-8kfr.mp4'
         },
         {
-            type: 'video/webm; codecs="vorbis"',
+            type: 'audio/webm; codecs="vorbis"',
             is_video: false,
             url: 'webm/test-a-128k-44100Hz-1ch.webm'
         },
         {
-            type: 'video/mp4; codecs="mp4a.40.2"',
+            type: 'audio/mp4; codecs="mp4a.40.2"',
             is_video: false,
             url: 'mp4/test-a-128k-44100Hz-1ch.mp4'
         },

third_party/WebKit/LayoutTests/external/wpt/media-source/mediasource-util.js

@@ -222,7 +222,10 @@
         return null;
     }
 
+    // To support mediasource-changetype tests, do not use any types that
+    // indicate automatic timestamp generation in this audioOnlyTypes list.
     var audioOnlyTypes = ['audio/mp4;codecs="mp4a.40.2"', 'audio/webm;codecs="vorbis"'];
+
     var videoOnlyTypes = ['video/mp4;codecs="avc1.4D4001"', 'video/webm;codecs="vp8"'];
     var audioVideoTypes = ['video/mp4;codecs="avc1.4D4001,mp4a.40.2"', 'video/webm;codecs="vp8,vorbis"'];
     MediaSourceUtil.AUDIO_ONLY_TYPE = getFirstSupportedType(audioOnlyTypes);