Allow plugins from extra plugin locations to be loaded multiple times.

This solves the issue where plugins are bundled with extensions and
might need to be loaded from more than one profile at the same time

BUG=289725
TEST=As described on the bug.

Review URL: https://codereview.chromium.org/40363002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231049 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231049