Replace string comparing to enum comparing.

since process() is executed frequently, enum comparing to find panningModel is
better than string comparing.

BUG=none

Review URL: https://codereview.chromium.org/231023005

git-svn-id: svn://svn.chromium.org/blink/trunk@171168 bbb929c8-8fbe-4397-9dbb-9b2b20218538