Oilpan: Build fix for r173101.

EventHandlerRegistry should be HeapSupplement<Page>.

BUG=
TBR=skyostil@chromium.org,oilpan-reviews@chromium.org

Review URL: https://codereview.chromium.org/262753008

git-svn-id: svn://svn.chromium.org/blink/trunk@173135 bbb929c8-8fbe-4397-9dbb-9b2b20218538