Oilpan: lazily create SelfKeepAlive<>'s persistent reference.
Oilpan: lazily create SelfKeepAlive<>'s persistent reference.

To reduce overhead of having to register a Persistent<> that may
only have a non-null reference for a shorter interval, instantiate
the Persistent<> on assignment. And clear it out completely when
the SelfKeepAlive<> is cleared.

R=haraken
BUG=420515

Review URL: https://codereview.chromium.org/1263313003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@200028 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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