Oilpan: make Supplementable tracing more regular.

With PersistentHeapSupplementable retired, follow up and only provide a
trace() method for a Supplementable that's on the heap.

No changes made for Supplements: an empty trace() method is still
provided for off-heap supplements, as this is the better solution
while we still are using the transition type WillBeHeapSupplement
for all the supplement objects.

R=haraken,zerny,wibling
BUG=395036

Review URL: https://codereview.chromium.org/587393002

git-svn-id: svn://svn.chromium.org/blink/trunk@182477 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Mark media/gc-while-playing.html as not flaky

It is no longer failing after r182442:
https://codereview.chromium.org/587373002/

BUG=400659
NOTRY=true

Review URL: https://codereview.chromium.org/593013002

git-svn-id: svn://svn.chromium.org/blink/trunk@182478 bbb929c8-8fbe-4397-9dbb-9b2b20218538