Speed-up whitelist check in data_pack.py

Previously whitelist was passed as a list causing a slow down when doing
'key in whitelist' check. Converting the list to a set to speed that up.

Local testing shows about 4x speedup in running RePack().

BUG=338759
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/244893005

Patch from Aurimas Liutikas <aurimas@chromium.org>.

git-svn-id: http://grit-i18n.googlecode.com/svn/trunk@166 7262f16d-afe8-6277-6482-052fa10e57b1