Oilpan: trace part objects as per the GC plugin's wishes.

Following https://src.chromium.org/viewvc/chrome?view=rev&revision=287310,
the Blink clang plugin insists that part objects are to be traced
via Visitor's fallback method

  template<typename T>void trace(const T& t);

Fix up some uses that were not.

R=haraken
BUG=340522
NOTRY=true

Review URL: https://codereview.chromium.org/471463002

git-svn-id: svn://svn.chromium.org/blink/trunk@180152 bbb929c8-8fbe-4397-9dbb-9b2b20218538