Dynamically generate a heading for Extension Docs API pages

chrome/common/extensions/api/alarms.idl

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
-
 // TODO(mpcomplete): We need documentation before we can release this.
 
+// Use the <code>chrome.alarms</code> module to schedule code to run
+// periodically or at a specified time in the future.
+
+[availability=22]

[not at google - send to devlin, 2013/04/02 10:29:39]

We should be able to determine this automatically right? The alarms API was
added post-extension_api.json split (in fact it's IDL so by definition it was
post-JSON).

And yet when I remove it it says availability is 18. Hm.

(Same comment applies to all IDL files, and probably some of the JSON ones too.)

[epeterson, 2013/04/03 00:36:35]

If you're using preview.py, then this patch is only interacting with old, "fake"
data, which leads to the generated availability being deemed as 18 for just
about everything (since the data must exist in the fake filesystem at version 18
and the hardcoded cutoff is at 18.

When using the appengine (along with logging messages), the generated
availability should be more accurate. 

[not at google - send to devlin, 2013/04/03 00:41:14]

Ah - makes sense, sorry about that. I was actually thinking the other day that
we need a flag like --use-live-data to preview.py for testing exactly this. But
let's do this after I get my refactor in.

 namespace alarms {
   dictionary Alarm {
     // Name of this alarm.
     DOMString name;
 
     // Time at which this alarm was scheduled to fire, in milliseconds past the
     // epoch (e.g. <code>Date.now() + n</code>).  For performance reasons, the
     // alarm may have been delayed an arbitrary amount beyond this.
     double scheduledTime;
 
@@ skipping 71 matching lines @@
     // Clears all alarms.
     static void clearAll();
   };
 
   interface Events {
     // Fired when an alarm has elapsed. Useful for event pages.
     // |alarm|: The alarm that has elapsed.
     static void onAlarm(Alarm alarm);
   };
 };