Fix memory leak in platfrom calc

Platfrom calcs were not being properly deref'd and were never freed
after being used in a Length object.
This patch corrects the deref behaviour and avoids the memory leak.

BUG=291498

Review URL: https://chromiumcodereview.appspot.com/23967013

git-svn-id: svn://svn.chromium.org/blink/trunk@157813 bbb929c8-8fbe-4397-9dbb-9b2b20218538