Oilpan: Fix ParamStorageTraits for RawPtr<T>.

RawPtr<> can be used for off-heap classes.  e.g. RawPtrWillBeMember<OffHeap>
So, we should check if T is on-heap for ParamStorageTraits<RawPtr<T>>.

This CL also introduces IsGarbageCollectedType<T> struct to simplify on-heap
class check.

BUG=

Review URL: https://codereview.chromium.org/322343002

git-svn-id: svn://svn.chromium.org/blink/trunk@175951 bbb929c8-8fbe-4397-9dbb-9b2b20218538