Cleanup: Make EventTarget::hasEventListeners const.

They should be const conceptually, and actually don't alter objects.
We stil have const_cast<> in hasEventListeners because eventTargetData
is not const. We'll address it later.

BUG=none
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/23532088

git-svn-id: svn://svn.chromium.org/blink/trunk@158249 bbb929c8-8fbe-4397-9dbb-9b2b20218538