[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
Suggest to rewrite to avoid the indent inside the if block
```
if (!runtime)
  return GURL();

scoped_refptr<base::RefCountedMemory> icon_bytes =
...
```