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This is phase 3 of a refactor that allows Select-to-Speak to work
with Sticky Keys, see go/sts-with-sticky-keys for more on the
background for this change.

Not only are EventHandlers ordered by priority across all EventTargets,
but also the later an EventHandler was added to its EventTarget, the
earlier it will appear in the resulting list of EventHandlers. This is
a change for kDefault handlers, but kAccessibility and kSystem already
had this behavior. This is for consistency across all types and requires
a large change to event_dispatcher_unittest.

Also adds an additional debugging function to TestEventHandler
to get that handler's name. This was useful in writing the unittests
for this change and may be useful to others as well.

Bug: 819860
Change-Id: I4c5002fdc2cd26f0df5e94600183c3cc0cc2ff56
Reviewed-on: https://chromium-review.googlesource.com/996456Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568557}