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This CL replaces all ::identity namespace with ::signin namespace
in //components/signin and updates all related codes.

TBR=jochen@chromium.org

Bug: 984884
Change-Id: I14a74f2288b0eee1c039881596461558bd27e4e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1712045
Commit-Queue: Miyoung Shin <myid.shin@igalia.com>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679997}