fix text sizing inaccuracy from 980px switch

The side margin was in CSS px, but goalWidth was in dp. This caused the
goalWidth to be smaller than it should, which caused scrollWidth to be
smaller, which caused each message's zoom factor to be larger, which
caused text to be larger than it used to be.

Now. goalWidth and documentWidth both have body margins accounted for in
the proper units.

Bug: 12579959
Change-Id: Ice36b48f381cf3171b092ba27152aa56b64f4100
5 files changed