Split up media into video, audio and track

This splits up the `media` destination into three distinct destinations:
`video`, `audio` and `track`.

The reason for that is that track is quite different from video and
audio, and at least in Chromium, treating it internally as
a media resource type is problematic.

This change would allow type specific checks for track, and enable
browsers to modify `Accept` headers according to accepted MIME types
for those different resource types.

fetch.bs

@@ -772,18 +772,20 @@ the empty string,
 <p>A <a for=/>request</a> has an associated
 <dfn export for=request id=concept-request-destination>destination</dfn>, which is
 the empty string,
+"<code>audio</code>",
 "<code>document</code>",
 "<code>embed</code>",
 "<code>font</code>",
 "<code>image</code>",
 "<code>manifest</code>",
-"<code>media</code>",
 "<code>object</code>",
 "<code>report</code>",
 "<code>script</code>",
 "<code>serviceworker</code>",
 "<code>sharedworker</code>",
 "<code>style</code>",
+"<code>track</code>",
+"<code>video</code>",
 "<code>worker</code>", or
 "<code>xslt</code>". Unless stated otherwise it is the empty string.
 
@@ -830,7 +832,7 @@ the empty string,
    <td>HTML's <code>&lt;embed></code>
   <tr>
    <td>"<code>audio</code>"
-   <td>"<code>media</code>"
+   <td>"<code>audio</code>"
    <td><code>media-src</code>
    <td>HTML's <code>&lt;audio></code>
   <tr>
@@ -869,12 +871,12 @@ the empty string,
    <td>HTML's <code>&lt;link rel=stylesheet></code>, CSS' <code>@import</code>
   <tr>
    <td>"<code>track</code>"
-   <td>"<code>media</code>"
+   <td>"<code>track</code>"
    <td><code>media-src</code>
    <td>HTML's <code>&lt;track></code>
   <tr>
    <td>"<code>video</code>"
-   <td>"<code>media</code>"
+   <td>"<code>video</code>"
    <td><code>media-src</code>
    <td>HTML's <code>&lt;video></code> element
   <tr>
@@ -1106,10 +1108,10 @@ Unless stated otherwise, it is "<code>basic</code>".
 <hr>
 
 <p>A <dfn export>subresource request</dfn> is a <a for=/>request</a>
-whose <a for=request>destination</a> is "<code>font</code>",
-"<code>image</code>", "<code>manifest</code>", "<code>media</code>",
-"<code>script</code>", "<code>style</code>", "<code>xslt</code>", or the empty
-string.
+whose <a for=request>destination</a> is "<code>audio</code>", "<code>font</code>",
+"<code>image</code>", "<code>manifest</code>", "<code>script</code>",
+"<code>style</code>", "<code>track</code>", "<code>video</code>",
+"<code>xslt</code>", or the empty string.
 
 <p>A <dfn export>potential-navigation-or-subresource request</dfn> is a
 <a for=/>request</a> whose
@@ -4627,7 +4629,7 @@ dictionary RequestInit {
 };
 
 enum RequestType { "", "audio", "font", "image", "script", "style", "track", "video" };
-enum RequestDestination { "", "document", "embed", "font", "image", "manifest", "media", "object", "report", "script", "serviceworker", "sharedworker", "style",  "worker", "xslt" };
+enum RequestDestination { "", "audio", "document", "embed", "font", "image", "manifest", "object", "report", "script", "serviceworker", "sharedworker", "style",  "track", "video", "worker", "xslt" };
 enum RequestMode { "navigate", "same-origin", "no-cors", "cors" };
 enum RequestCredentials { "omit", "same-origin", "include" };
 enum RequestCache { "default", "no-store", "reload", "no-cache", "force-cache", "only-if-cached" };