Do not check for NULL pointers before calling partitionFreeGeneric().

It is no longer necessary to perform this check, it's handled internally in the
allocator. And it will continue to be handled with the ongoing changes.

This shaves 7KB off the release Chrome binary.

Review URL: https://codereview.chromium.org/123103002

git-svn-id: svn://svn.chromium.org/blink/trunk@164416 bbb929c8-8fbe-4397-9dbb-9b2b20218538