Oilpan: Fix tracing of events through XMLHttpRequestProgressEventThrottle.

This was found with the clang Blink GC plugin. Let's all turn it on
and keep it running! :)

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

Review URL: https://codereview.chromium.org/223163004

git-svn-id: svn://svn.chromium.org/blink/trunk@170754 bbb929c8-8fbe-4397-9dbb-9b2b20218538