Disabled external_extensions.json in Guest mode.

Added browser_defaults::extensions_enabled to control this behavior.

BUG=chromiumos:4420
TEST=Go to Guest mode and verify that no extensions are loaded by going to chrome://extensions.

Review URL: http://codereview.chromium.org/3462016

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@60759 0039d316-1c4b-4281-b951-d872f2087c98