Oilpan: Expose Heap::collectGarbage to the web layer

Heap::collectGarbage is needed in media_stream_impl_unittest.

BUG=340522

Review URL: https://codereview.chromium.org/543603003

git-svn-id: svn://svn.chromium.org/blink/trunk@181615 bbb929c8-8fbe-4397-9dbb-9b2b20218538