Refactor startup flags and enable viewport and touch feedback

The refactor makes the code similar to how it is done for Chrome for
Android and enables viewport (incl required fixed layout) and feedback
on touch.

It has been tested on desktop and works fine with the following:

xwalk "data:text/html,<meta name='viewport' content='width=320'>" \
  "<body><div style='width:300px; background-color:red;'> .</div></body>"