Callbacks can keep embedder objects alive, hence clear them after
delivering the final event.

R=ahaas@chromium.org

Bug: chromium:912764
Change-Id: I9ac739bbce32cb1026991610e0720210717c333e
Reviewed-on: https://chromium-review.googlesource.com/c/1371565
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58168}