Oilpan: Prepare to make ExecutionContext GarbageCollectedMixin.

We creates closures with ExecutionContexts in LocalFileSystem. The
ExecutionContext needs to be a CrossThreadPersistent data member of
closure. This CL adds ParamStorageTraits specialized to garbage-collected
classes.

This is a preparation to remove RefCounted from Node.

BUG=357163

Review URL: https://codereview.chromium.org/318023002

git-svn-id: svn://svn.chromium.org/blink/trunk@175734 bbb929c8-8fbe-4397-9dbb-9b2b20218538