Remove webkitNotifications.createHTMLNotification

This CL removes support for webkitNotifications.createHTMLNotification. This
API no longer works on any platform except Linux. We haven't exposed this API
to the web in a long time, but we've been keeping it around to support a
handful of Chrome extensions that were using the feature. These extensions have
now migrated to Rich Notifications
<http://blog.chromium.org/2013/05/rich-notifications-in-chrome.html>. As we've
implemented Rich Notifications on various platforms, we've been removing
support for createHTMLNotification. At this point, createHTMLNotification works
only on Linux.

This CL does not affect the text-based webkitNotifications.createNotification
or the unprefixed "new Notification" APIs, both of which will continue to
function after this CL.

BUG=137297

Review URL: https://chromiumcodereview.appspot.com/21087006

git-svn-id: svn://svn.chromium.org/blink/trunk@155187 bbb929c8-8fbe-4397-9dbb-9b2b20218538