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
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=200028

[sof, 2015-08-04 21:06:17]

sigbjornf@opera.com changed reviewers:
+ oilpan-reviews@chromium.org

[sof, 2015-08-04 21:06:18]

please take a look.

having a smaller persistent root set is in our interest, so it would be
preferable to lazily create these self keep-alives persistents, even if it adds
an OwnPtr<> allocation indirection.

[haraken, 2015-08-04 23:16:12]

LGTM

[sof, 2015-08-05 05:09:38]

The CQ bit was checked by sigbjornf@opera.com

[commit-bot: I haz the power, 2015-08-05 05:09:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1263313003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1263313003/1

[commit-bot: I haz the power, 2015-08-05 06:04:58]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=200028