Make default size consistent in code

This patch makes a tiny adjustment to three files so that the default size is referred to consistently in code as 640x360, which is the 16:9 aspect ratio.

Changes to video_wysiwyg.js:
height: 420
height: 360

Changes to video.preset.inc:
'#default_value' => !empty($settings['wxh']) ? $settings['wxh'] : '640x480',
'#default_value' => !empty($settings['wxh']) ? $settings['wxh'] : '640x360',

Changes to video.css:
height: 325px;
height: 360px;