Split out Members, Persistents and SelfKeepAlive in separate headers.

Complete the migration of definitions out of Handle.h, adding new
header files for the family of Member<> types that Oilpan supports.
Similarly for Persistent<> and its variants, and the SelfKeepAlive<>
abstraction.

To further prune Handle.h, let go of the unused ScopedDisposal (light)
abstraction.

R=
BUG=

Review-Url: https://codereview.chromium.org/1999363002
Cr-Commit-Position: refs/heads/master@{#395318}
15 files changed