Oilpan: fix build after r202130.

Oilpan: fix build after r202130.

The Blink GC plugin is syntactic, requiring GCed fields to literally
appear within the trace() method body. Once that don't are assumed not
to have been traced. Which prevents the use of an accessor, like r202130
introduced.

TBR=oilpan-reviews,yosin
BUG=513568
NOTRY=true

Committed: https://crrev.com/dac6dd3eac24e901b01d2e6f0fb9fa015f8f1b82
git-svn-id: svn://svn.chromium.org/blink/trunk@202137 bbb929c8-8fbe-4397-9dbb-9b2b20218538

[sof, 2015-09-11 14:40:18]

The CQ bit was checked by sigbjornf@opera.com

[sof, 2015-09-11 14:40:22]

The CQ bit was unchecked by sigbjornf@opera.com

[sof, 2015-09-11 14:40:31]

The CQ bit was checked by sigbjornf@opera.com

[commit-bot: I haz the power, 2015-09-11 14:40:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1337933002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1337933002/1

[commit-bot: I haz the power, 2015-09-11 14:45:30]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=202137

[haraken, 2015-09-11 15:38:43]

LGTM

[commit-bot: I haz the power, 2015-09-23 12:22:22]

Patchset 1 (id:??) landed as
https://crrev.com/dac6dd3eac24e901b01d2e6f0fb9fa015f8f1b82