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This avoids having tests defining their own delegate implementation
when they need one.

Bug: 1021600
Change-Id: I12d84c647e17c3c476ce2dc19f5298a524cf98dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1900346
Commit-Queue: Patrick Monette <pmonette@chromium.org>
Reviewed-by: Martin Šrámek <msramek@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714749}