Add AtomicString, SkString version of equalIgnoringCase

Add a version of equalIgnoringCase that takes an AtomicString and a
SkString and use it for font family matches. This takes care of the
encoding logic and ensures that the easiest way to compare the two
is the right way.

R=eseidel@chromium.org,leviw@chromium.org
BUG=249099

Review URL: https://chromiumcodereview.appspot.com/23835013

git-svn-id: svn://svn.chromium.org/blink/trunk@158042 bbb929c8-8fbe-4397-9dbb-9b2b20218538