Oilpan: Force only one Heap::collectGarbage() in gcEpilogue()

Currently GCController.collectAll() triggers 7 V8's GC, each of which triggers 5 Heap::collectGarbage(). This is too much and slowing down layout tests.

This CL makes a change so that one V8's GC triggers only one Heap::collectGarbage(). I think this is enough in practice.

BUG=340522

Review URL: https://codereview.chromium.org/228663003

git-svn-id: svn://svn.chromium.org/blink/trunk@171118 bbb929c8-8fbe-4397-9dbb-9b2b20218538