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This change converts CredentialManagerProxy to use
BrowserInterfaceBroker, along with mocks in tests.

Bug: 995422
Change-Id: I466ea95239108af5b106651adf18517b23828717
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1808300
Commit-Queue: Oksana Zhuravlova <oksamyt@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699948}