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When an expectations file for accessibility dump tree or event tests
has more lines than the actual output, the test must fail.

Without this change, the following will produce false positives
in our test results:
- Expected events the end of event expectations, but are not fired,
- Expected objects at the end of the object hierarchy that do not
exist in the actual object hierarchy.

Bug: None
Change-Id: I4bb65bb73b0d8af7380c85e59c7891c466c95fc0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015429
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Auto-Submit: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735749}