Use user specific NSSDatabase in CertLoader.

CertLoader is still global object, but it now loads only primary
user's certificates. Loading only primary user's certificates
is ok, since shill only uses primary user's network profile
(and currently only network stack is interested in certificates from CertLoader).

Added some tests for CertLoader and NetworkConnectionHandler.

BUG=315343

Review URL: https://codereview.chromium.org/135193007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247414 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247414