A separate CL adds a test stanza to the metrics ebuild. More tests
(specifically, for the D-Bus MessageFilter) will also come in a
separate CL.
Review URL: http://codereview.chromium.org/1919005
Add restart_in_chroot_if_needed to common.sh, and modify the build scripts which referred to assert_inside_chroot to use it instead. The effect is that you don't ever have to explicitly enter_chroot.sh to build (still can, it work's fine).
Update mod_image_for_test.sh to use restart_in_chroot_if_needed
Review URL: http://codereview.chromium.org/1736025
