Start with event_target in for loop to avoid GetEventTargeter crash.

Came across this while working on the CL for dispatching event to root
window if target window has already been deleted ( If the target window we
are starting from is already the root window in that WindowTreeHost,
starting with the parent target (in this case aura::Env) for that window
would cause crash when trying to access its event targeter. Change it to
start with the event_target itself.

BUG=687700, 664617

Cr-Commit-Position: refs/heads/master@{#454780}
2 files changed