Have PlatformEventObserver implementations orderly stop on destruction.

Move out the stop-a-still-observing-observer-on-destruct handling to the
destructors of the objects that implement the pure virtual
SendStopMessage().

Cannot call upon a virtual from within ~PlatformEventObserver.

R=jochen@chromium.org,mkwst@chromium.org,mlamouri@chromium.org
BUG=

Review URL: https://codereview.chromium.org/613203002

Cr-Commit-Position: refs/heads/master@{#297614}
7 files changed