Removing the remaining unused RoundedRectStrategy code

As suggested in review, the RoundedRect strategy was a code path only used
by WebKit Mac port and could be removed in other places in Path class (and its
callers).

BUG=260260

Review URL: https://chromiumcodereview.appspot.com/19434002

git-svn-id: svn://svn.chromium.org/blink/trunk@154338 bbb929c8-8fbe-4397-9dbb-9b2b20218538