Teach grit that file.svg is an SVG.

When dropping the reliance of platform mime databases we also lost
support for the svg mime type. Readding it specifically.

Also added an explicit failure if something similar happens in the future.

BUG=383189
R=newt@chromium.org

Review URL: https://codereview.chromium.org/324383003

git-svn-id: http://grit-i18n.googlecode.com/svn/trunk@169 7262f16d-afe8-6277-6482-052fa10e57b1