Load external extensions even when --disable-extensions is used. We do this since different environments may require users to have certain external policy extensions installed. This patch also fixes a crash when --disable-extensions is used with external extensions installed.

BUG=66070
TEST=ExtensionsServiceTest.ExternalUninstall

Review URL: http://codereview.chromium.org/5695004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@68892 0039d316-1c4b-4281-b951-d872f2087c98