ash: Add --ash-default-wallpaper-is-oem switch.

Add an --ash-default-wallpaper-is-oem switch, which should
be set when the default wallpaper images are OEM-specific.
This will eventually replace the current
--ash-oem-wallpaper-{large,small} switches.

BUG=315267

Review URL: https://codereview.chromium.org/62063004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233624 0039d316-1c4b-4281-b951-d872f2087c98
Make the HMACs actually depend on device_id and pref_path.

Added unit tests to prove the mistake (ran without this fix as try jobs on https://codereview.chromium.org/62573003/).

R=asvitkine@chromium.org, battre@chromium.org

Review URL: https://codereview.chromium.org/63033002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233625 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233625