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This change makes PublicKeyCredentialDescriptor credential_type to be of
enum type CredentialType instead of std::string, increasing type-safety.

Bug: 780078
Change-Id: I570e698ecb9b13d7cc988eb52293a65f466c8115
Reviewed-on: https://chromium-review.googlesource.com/1041951
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558274}