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This was an abstraction so that tests could substitute in a fake
version of a Mojo interface in tests. However, the test can just
implement a testing version of the Mojo interface and use it
directly, simplifying the code.

BUG=none

Review-Url: https://codereview.chromium.org/2345423002
Cr-Commit-Position: refs/heads/master@{#420583}