Have grit survive a Windows registry with Unicode keys.

grit triggered a python bug http://bugs.python.org/issue9291 that
broke the build. This change is to rely only on the internal
python mime type list which contains everything we need anyway.

BUG=chromium:365188
Review URL: https://codereview.chromium.org/253443002/

Patch from Daniel Bratell <bratell@opera.com>!


git-svn-id: http://grit-i18n.googlecode.com/svn/trunk@168 7262f16d-afe8-6277-6482-052fa10e57b1