make sure fields are initialized in OverflowEventDispatcher
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@171094 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs