StyleDifference should store property-specific difference in a bit field

This CL prepares us for adding a hasOnlyPropertySpecificDifference function in
a future CL that operates on all the property-specific differences as a unit.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/361283002

git-svn-id: svn://svn.chromium.org/blink/trunk@177417 bbb929c8-8fbe-4397-9dbb-9b2b20218538