[Download] Fix and complete Download API

Refactored Filesytem and Download API to share code
related to virtual roots.
Pass TCT tests.
Fix example.

BUG=XWALK-1070
Without 'static', I've got some build errors when this const variable is used in filesystem extension.

Btw, exception here : http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml#Access_Control
makes me think that a static const data member in VirtualFS class is fine.

If it is not, can you help me to understand ?