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This CL expands on the work to include an AccessibilityEventDispatcher
on Android, relevant CL:
https://chromium-review.googlesource.com/c/chromium/src/+/2139905

Here we include a simple set of tests to check our queueing logic
and throttling of specific event types. We test for both events that
are and are not throttled, with cases of the first event, an event
after some time with no events, and events in quick succession.

We also slightly modified the AccessibilityEventDispatcher with this
CL to fix a dependency injection issue to make the testing easier.

Change-Id: I13aed5541eba0feaf1b250bf15cae9809efe54b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2151851Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Reviewed-by: Mark Schillaci <mschillaci@google.com>
Commit-Queue: Mark Schillaci <mschillaci@google.com>
Cr-Commit-Position: refs/heads/master@{#759831}