Setup a full blink test environment for blink_heap_unittests target.

This makes Platform::current available in the blink heap unit tests.
That allows us to use the threading APIs available through the
Platform object. Those threading APIs are used internally in the
Oilpan infrastructure as well for parallel marking/sweeping and
therefore we can now write tests of that multi-threaded code as
part of the unit tests.

No change in the tests in this patch. Updates to use the Platform
threading APIs will follow.

R=jochen@chromium.org, wibling@chromium.org, zerny@chromium.org, oilpan-reviews@chromium.org
BUG=416376

Review URL: https://codereview.chromium.org/587253002

git-svn-id: svn://svn.chromium.org/blink/trunk@182425 bbb929c8-8fbe-4397-9dbb-9b2b20218538