Add support for allocating large objects.

I had to disable forced gc collections since the large object tests triggered it before having been assigned to a persistent. Since we don't support conservative stack scanning yet the large object got incorrectly collected.

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

Review URL: https://codereview.chromium.org/130913005

git-svn-id: svn://svn.chromium.org/blink/trunk@164782 bbb929c8-8fbe-4397-9dbb-9b2b20218538