Fix calc length memory leak

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://codereview.chromium.org/25677002

git-svn-id: svn://svn.chromium.org/blink/trunk@158909 bbb929c8-8fbe-4397-9dbb-9b2b20218538