Remove the PAN_SCROLLING feature define

Pan scrolling is Windows-only, so remove this feature define and
replace it with a check for OS(WINDOWS).

BUG=246664

Review URL: https://chromiumcodereview.appspot.com/18408004

git-svn-id: svn://svn.chromium.org/blink/trunk@154198 bbb929c8-8fbe-4397-9dbb-9b2b20218538