Oilpan: FormDataList should be GarbageCollected

Currently FormDataList has a trace method (which traces FormDataList::m_items)
but is not GarbageCollected. This means that the trace method is not working.
(I wonder why the current code is working without crashing.)

This CL makes FormDataList GarbageCollected and makes sure that FormDataList::m_items
is traced.

BUG=340522

Review URL: https://codereview.chromium.org/592823002

git-svn-id: svn://svn.chromium.org/blink/trunk@182542 bbb929c8-8fbe-4397-9dbb-9b2b20218538