ScriptPromisePropertyTest: Invoke Oilpan GC in memory-related tests too.

BUG=391630

Review URL: https://codereview.chromium.org/370193004

git-svn-id: svn://svn.chromium.org/blink/trunk@177641 bbb929c8-8fbe-4397-9dbb-9b2b20218538