[Tizen][Linux] Implement the DevTools faviconUrl.

The favicon is stored in Runtime::app_icon(), so in DevTools we get it and covert to "data:" url.

BUG=https://crosswalk-project.org/jira/browse/XWALK-2858
you hard code image with png, is that saying we only support png favicon? If no, you need identify the type instead of hard code.