Reset plugin group version to "0" instead of NULL.
Reset plugin group version to "0" instead of NULL.

This guards against dereferencing NULL when the group is empty.

BUG=72946
TEST=PluginGroupTest.*

Review URL: http://codereview.chromium.org/6532024

git-svn-id: http://src.chromium.org/svn/trunk/src@75270 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: d109b0d503e51c7fceb3bf21ea67be309df8d7f8
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser