Add implementation of Oilpan Heap::allocate.

We are getting closer to being able to put in the GarbageCollected
super class so we can get the first tests running that allocate
objects in the heap.

R=erik.corry@gmail.com, vegorov@chromium.org, wibling@chromium.org, zerny@chromium.org
BUG=

Review URL: https://codereview.chromium.org/111743004

git-svn-id: svn://svn.chromium.org/blink/trunk@164181 bbb929c8-8fbe-4397-9dbb-9b2b20218538