Oilpan: Flush a HeapDoesNotContainCache before calling checkAndMarkPointer

This is a speculative fix for issue 420699.

We must flush a HeapDoesNotContainCache before calling checkAndMarkPointer. This condition holds in the production system because checkAndMarkPointer is called only during a GC and we flush a HeapDoesNotContainCache before starting the GC (if needed). The only place where the condition doesn't hold is a unittest in HeapTest.cpp. This CL adds code to flush the HeapDoesNotContainCache in the unittest.

BUG=420699

Review URL: https://codereview.chromium.org/644563002

git-svn-id: svn://svn.chromium.org/blink/trunk@183531 bbb929c8-8fbe-4397-9dbb-9b2b20218538