Add GCScope object to the oilpan heap infrastructure.

Sweep is still not implemented. Instead of actually sweeping we
clear all marks. Implementing actual sweeping is next on the list.

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

Review URL: https://codereview.chromium.org/128533002

git-svn-id: svn://svn.chromium.org/blink/trunk@164756 bbb929c8-8fbe-4397-9dbb-9b2b20218538