Oilpan: Fix tracing of KeyframeVector in CompositorAnimationsTest.

R=haraken@chromium.org, oilpan-reviews@chromium.org
BUG=

Review URL: https://codereview.chromium.org/183923038

git-svn-id: svn://svn.chromium.org/blink/trunk@168617 bbb929c8-8fbe-4397-9dbb-9b2b20218538