Revert 157813 "Fix memory leak in platfrom calc"

Caused several use-after-frees on ClusterFuzz.

> 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

TBR=alancutter@chromium.org

Review URL: https://codereview.chromium.org/23483056

git-svn-id: svn://svn.chromium.org/blink/trunk@158035 bbb929c8-8fbe-4397-9dbb-9b2b20218538