Revert 170754 "Oilpan: Fix tracing of events through XMLHttpRequ..."

r170733 broke a lot of tests in oilpan bots.
To revert r170733, we need to revert r170754.

> 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

TBR=ager@chromium.org

Review URL: https://codereview.chromium.org/223073005

git-svn-id: svn://svn.chromium.org/blink/trunk@170755 bbb929c8-8fbe-4397-9dbb-9b2b20218538