[cros] Pass pepper plugin flag for Guest mode quoted.

This change will only work after session_manager change is submitted:
http://codereview.chromium.org/4734002

BUG=chromium-os:7526
TEST=Manual. Flash works in Guest mode.

Review URL: http://codereview.chromium.org/4743001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@65669 0039d316-1c4b-4281-b951-d872f2087c98