Make calls to AtomicString(const String&) explicit in css/

Make calls to AtomicString(const String&) explicit in css/ and avoid useless
conversions from String to AtomicString when possible.

This is one step towards making the AtomicString(const String&) constructor
explicit. This CL takes care only of the css/ part so that it is more
easily reviewable.

R=eseidel, abarth
BUG=323739

Review URL: https://codereview.chromium.org/100463004

git-svn-id: svn://svn.chromium.org/blink/trunk@163461 bbb929c8-8fbe-4397-9dbb-9b2b20218538