Implement sweeping for the blink garbage collection infrastructure
and update tests to actually perform allocations and garbage collections.

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

Review URL: https://codereview.chromium.org/131353002

git-svn-id: svn://svn.chromium.org/blink/trunk@164765 bbb929c8-8fbe-4397-9dbb-9b2b20218538