Fix memory leak in platfrom calc
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@157813 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs