Use css to initially scale on zoom.

Also:
-fixes the b2g viewer by adding the missing stubs
-adds an option to only use css zoom and draw the canvas at 100% width (can be tested with #useCssZoom=true)
-I think we were storing this.scale somewhat inconsistently, so I refactored the viewer to always store this.scale in the pdf scale units and only adjust it by CSS_UNITS when it is passed into the viewport.

Fixes:
https://bugzilla.mozilla.org/show_bug.cgi?id=845083