Oilpan: fix tracing of KeyframeVector which is a HeapVector of Members.

The HeapVector was put in a normal Vector of pairs on the stack
which is not traced.

This issue was found with the ASAN build on layout tests.

R=erik.corry@gmail.com, haraken@chromium.org, wibling@chromium.org, zerny@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/180273021

git-svn-id: svn://svn.chromium.org/blink/trunk@168386 bbb929c8-8fbe-4397-9dbb-9b2b20218538