EventSender<T> singletons are better off on the Oilpan heap.

Rather than keeping two off-heap persistent collections per EventSender
singleton, have the singletons reside on the Oilpan heap instead.

This also removes no-op cancelEvent()s from various destructors.

R=haraken
BUG=420515

Review URL: https://codereview.chromium.org/1554903002

Cr-Commit-Position: refs/heads/master@{#367282}
17 files changed