Couple of tuning for the Browser zoom.

1. Added a setting to control whether a page is loaded with overview mode;
2. If there is no viewport metag tag, a page will be loaded in the viewport
   at least 800px wide.
3. When we adjust zoom scale in the overview mode, don't use animation.
4. When zoom out to close to min zoom scale, switch to overview mode. So double
   tap will always have visual feedback unless it is mobile site.