Delete now unused StyleDifferenceContextSensitiveProperty enum

After https://src.chromium.org/viewvc/blink?view=rev&revision=177358
these enumeration values are unused.

Review URL: https://codereview.chromium.org/366813004

git-svn-id: svn://svn.chromium.org/blink/trunk@177391 bbb929c8-8fbe-4397-9dbb-9b2b20218538