Stop using `assert_exists` and `assert_not_exists`

These have been removed in wpt's testharness.js:
https://github.com/web-platform-tests/wpt/pull/12976

This is blocking updating LayoutTests/resources/testharness.js.

`assert_exists` was blindly replaced with `assert_own_property` in all
files (s/assert_own_property/assert_exists/g) and `assert_not_exists`
was changed on a case-by-case basis.

Bug: 892613
Change-Id: Ied0d7952c5737b5afe50088cdd8be2c9629d40b7
Reviewed-on: https://chromium-review.googlesource.com/c/1264641
Reviewed-by: Robert Ma <robertma@chromium.org>
Commit-Queue: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597310}
63 files changed