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Measures the amount of time spent unserializing changes to the
accessibility tree and sending automation events.

In order to scale reasonably well across different machines, this
time is compared to the amount of time spent in renderer
accessibility code, that way the test only fails if the
disparity is too large.

The test code is deliberately all one test for ease of review.
As we add more tests, a lot of the code could be broken up into
helpers.

Bug: none
Change-Id: I44a169a7c80d9ccea5cf70cb7450731a43638c7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2430304
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811534}