[ozone/platform/wayland] Call OnNativeWidgetDestroying on destruction.

DesktopWindowTreeHostPlatform must notify native widget delegate
about destruction. This allows to properly release resources,
and fixes a AccessebilityChecker in browser_tests.

Bug: 578890
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I316fda5a2b2f35c865e192fc12274bc57cc2f1bf
Reviewed-on: https://chromium-review.googlesource.com/1059208
Commit-Queue: Maksim Sisov <msisov@igalia.com>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559839}
18 files changed