Oilpan: fix build after r174943.

Add missing tracing of added Event member.

R=haraken@chromium.org
BUG=376349
NOTRY=true

Review URL: https://codereview.chromium.org/303703004

git-svn-id: svn://svn.chromium.org/blink/trunk@174950 bbb929c8-8fbe-4397-9dbb-9b2b20218538