Updated hover media feature to comply updated media queries spec

The spec (http://dev.w3.org/csswg/mediaqueries-4/#hover)  for the hover
media query has added keywords for the feature's value. This CL updates
the media query keywords to propertly parse and evaluate the feature;
however, it remains only sparsely implemented. The hover (and pointer)
media queries currently only produce non-false output for touchscreen
devices. The plumbing work to actually produce the correct results in
all cases will be done in a future CL.

BUG=136119

Review URL: https://codereview.chromium.org/451113002

git-svn-id: svn://svn.chromium.org/blink/trunk@180006 bbb929c8-8fbe-4397-9dbb-9b2b20218538