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: svn://svn.chromium.org/blink/trunk@200028 bbb929c8-8fbe-4397-9dbb-9b2b20218538
1 file changed