Make Permissions.request() only take a single descriptor.

This matches the current Chromium implementation
(https://code.google.com/p/chromium/codesearch/#chromium/src/third_party/WebKit/Source/modules/permissions/Permissions.idl),
which has a requestAll() function to handle a sequence of permissions. A
sequence could also be handled with Promise.all(), like query() suggests.

index.html

@@ -611,7 +611,7 @@ <h2>
         interface Permissions {
           Promise&lt;PermissionStatus&gt; query(PermissionDescriptor permission);
 
-          Promise&lt;PermissionStatus&gt; request((PermissionDescriptor or sequence&lt;PermissionDescriptor&gt;) permissions);
+          Promise&lt;PermissionStatus&gt; request(PermissionDescriptor permission);
 
           Promise&lt;PermissionStatus&gt; revoke(PermissionDescriptor permission);
         };