In https://codereview.chromium.org/2540573002/ I fixed some edge case crashes where we enter event handling while a frame is detaching. We have these kinds of checks littered throughout event handling code. These checks are inconsistent and redundant. We'd be better off having a single entry point for all events where we can verify the system is capable of event handling (i.e. FrameView exists, Frame is attached, Document exists, etc.) and removing all these checks downstream.