make sure fields are initialized in OverflowEventDispatcher

This makes sure some older compilers don't complain that these member variables might be used uninitialized.

Review URL: https://codereview.chromium.org/212783002

git-svn-id: svn://svn.chromium.org/blink/trunk@171094 bbb929c8-8fbe-4397-9dbb-9b2b20218538