Oilpan: trace Event.currentTarget.

Strongly keep track of the current target.

For tidiness, also trace the 'parent' back reference on the XHR progress
event throttling part object.

R=zerny@chromium.org,ager@chromium.org
BUG=340522

Review URL: https://codereview.chromium.org/330273002

git-svn-id: svn://svn.chromium.org/blink/trunk@176112 bbb929c8-8fbe-4397-9dbb-9b2b20218538