[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
The png is the data sent to front-end, rather than favicon. The real favicon is stored in Runtime::app_icon and it supports multi-formats. We just read the raw data and send it in a convinient format.