Update FooEvent* arguments of functions called by DefaultEventHandler()

Update FooEvent* arguments of functions called by DefaultEventHandler()
in core/html/.
The arguments of functions never be nullptr, so it need to be reference
instead of pointer.

Bug: 874385
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I65925d4cf194482dd7216347fcbbec15e7f31089
Reviewed-on: https://chromium-review.googlesource.com/1180627
Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Byoungkwon Ko <codeimpl@gmail.com>
Cr-Commit-Position: refs/heads/master@{#584352}
12 files changed