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Before this CL, UserEventEqualityChecker would modify its
|expected_specifics_| in IsExitConditionSatisfied(), which usually gets
called multiple times. In the specific situations where it did this, it
was actually safe, but this was quite non-obvious.
The new version just makes a copy and modifies that. It's slightly more
verbose, but much less confusing.

Bug: none
Change-Id: I44b0220197625b168ff7394ec83ad7682e7b2d6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1795824
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695219}