The popup menu in the stocks app is supposed to size its width to the max
intrinsic width of the menu. This CL teaches it how to do that.

It's a shame that we need to ceilToDouble the output of RenderParagraph. If we
don't do that, we run into floating point layout trouble and the menu triggers
a line break. The correct fix is to do layout in fixed point.

R=ianh@google.com

Review URL: https://codereview.chromium.org/1168113005