mediawiki.hidpi: Add skipFunction to forgo loading in modern browsers

Chromium 34 (stable release April 8, 2014) introduced support
for the srcset attribute.

Chroem 34+ and Opera 21+ support this now. While the jquery.hidpi
module doesn't emulate srcset in browser that support it, we do
still load it.

http://blog.chromium.org/2014/02/chrome-34-responsive-images-and_9316.html
http://googlechromereleases.blogspot.com/2014/04/stable-channel-update.html
http://caniuse.com/srcset

Change-Id: I0498c9cc0f25e3d366879d9d3430ee8371efcf11

resources/Resources.php

@@ -865,6 +865,7 @@ return array(
 		'dependencies' => array(
 			'jquery.hidpi',
 		),
+		'skipFunction' => 'resources/src/mediawiki.hidpi-skip.js',
 		'targets' => array( 'desktop', 'mobile' ),
 	),
 	'mediawiki.hlist' => array(

resources/src/mediawiki.hidpi-skip.js

@@ -0,0 +1,4 @@
+/*!
+ * Skip function for mediawiki.hdpi.js.
+ */
+return 'srcset' in new Image();