Use more consistent background image scaling

Chrome in particular would end up with fairly small and inconsistent
background sizes when using background-size: contain;. This makes sense
according to the docs at
https://developer.mozilla.org/en-US/docs/Web/CSS/background-size, which
doesn't specify that the background actually has to fill the entire element.

background-size: 100% 100%; does force full-size scaling, which is what we
want. Verified to work as expected in Chrome 38 on Linux.

Change-Id: I508e08dac1480491efcd1d27cfac16187bfbd4fe
Merge "Use more consistent background image scaling"