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This makes it possible to use the types it contains in non-Blink
embedders such as iOS. The blink::WebCredential constructor of
CredentialInfo is now a factory function that stayed in content/.

BUG=470815
TBR=jochen

Review URL: https://codereview.chromium.org/1031253003

Cr-Commit-Position: refs/heads/master@{#322376}