Fix bug with context menus in incognito mode.

If an extension uses "spanning" mode, the context menu items are shared
between profiles. If an extension uses "split" mode, the items are separate
per profile. In either case, they only appear in incognito if the extension is
enabled in incognito.

Also fixed a minor bug, so that tabs.create now can open extension URLs in
incognito if the extension uses split mode.

BUG=61147
TEST=see bug for repro steps; context menu items should work in incognito mode

Review URL: http://codereview.chromium.org/4090011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@64812 0039d316-1c4b-4281-b951-d872f2087c98