Comment 1 for bug 1223946

I verified that the back button has the same dimensions as the other chrome buttons (forward and activity): 5×5 grid units.
As far as I know, that’s also the default dimensions of standard toolbar buttons in other applications.
The only difference is the left margin, which in the browser is 1 grid unit, and in the default toolbar for other applications is 2 grid units. I’ll make it 2 grid units for consistency, and I suspect this will fix the problem.