Make inline SVG fill parent element by default.
Otherwise `height` and `width` properties are being set to `auto`, which
computes to `150px` and `300px` in accordance to spec, which in turn
causes hover area bug.

Specification:
http://www.w3.org/TR/CSS2/visudet.html#inline-replaced-height

Relevant Bugzilla entry:
https://bugzilla.mozilla.org/show_bug.cgi?id=736431