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Currently, there are three independent testing implementations of the
CredentialsFilter API, all providing similar no-op implementation. In a coming
CL there will be a fourth one.

This CL pulls the stub of the API into a separate file, simplifying the 3
(soon 4) places in code where it is needed.

BUG=621355

Review-Url: https://codereview.chromium.org/2260203004
Cr-Commit-Position: refs/heads/master@{#413455}