Whitelist extensions from EXTERNAL_COMPONENT for public sessions

Extensions whose install location is EXTERNAL_COMPONENT are components of
Chrome itself and should be allowed in public sessions as well.

BUG=332512
TEST=Updated unit tests; manual

Review URL: https://codereview.chromium.org/141703003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246868 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246868