Bug 901712 - black boxing doesn't work with source maps; r=dcamp, a=akeybl
Bug 896547 - Fix regression due to incorrect change in 2765ec8ccf3b. r=Cwiiis a=akeybl

We use two different "pageWidth" calculations in browser.js, and the
regressing change switched from using one definition to the other. This
caused an infinite remeasure loop.