Parse media keys for named command in the manifest.

chrome/common/extensions/docs/templates/intros/commands.html

 <h2 id="manifest">Manifest</h2>
 <p>
 You must set manifest_version to (at least) 2 to use this API.
 </p>
 
 <h2 id="usage">Usage</h2>
 <p>The commands API allows you to define specific commands, and bind them to a
 default key combination. Each command your extension accepts must be listed in
 the manifest as an attribute of the 'commands' manifest key. An extension can
 have many commands but only 4 suggested keys can be specified. The user can
 manually add more shortcuts from the chrome://extensions page.</p>
 
 <p>Supported keys: A-Z, 0-9, Comma, Period, Home, End, PageUp, PageDown, Insert,
-Delete, Tab and the arrow keys (Up, Down, Left, Right).</p>
+Delete, Tab, Arrow keys (Up, Down, Left, Right) and the Media Keys
+(MediaNextTrack, MediaPlayPause, MediaPrevTrack, MediaStop).</p>
 
 <p>Note: All key combinations must include either Ctrl* or Alt. Combinations
 that involve Ctrl+Alt are not permitted in order to avoid conflicts with the
-AltGr key. Shift can be used in addition to Alt or Ctrl, but is not required.<p>
+AltGr key. Shift can be used in addition to Alt or Ctrl, but is not required.
+Modifiers (such as Ctrl) can not be used in combination with the Media Keys.<p>
 
 <p>* Also note that on Mac 'Ctrl' is automatically converted to 'Command'. If
 you want 'Ctrl' instead, please specify 'MacCtrl'.</p>
 
 <p>Certain Chrome shortcuts (e.g. window management) always take priority over
 Extension Command shortcuts and can not be overwritten.</p>
 
 <pre>{
   "name": "My extension",
   ...
@@ skipping 33 matching lines @@
   console.log('Command:', command);
 });
 </pre>
 
 <p>The '_execute_browser_action' and '_execute_page_action' commands are
 reserved for the action of opening your extension's popups. They won't normally
 generate events that you can handle. If you need to take action based on your
 popup opening, consider listening for an 'onDomReady' event inside your popup's
 code.
 </p>