Remove the obsolete avatarURL method from the credential manager.
Remove the obsolete avatarURL method from the credential manager.

As specified in https://w3c.github.io/webappsec/specs/credentialmanagement/#credential

This CL is a part of the three-sided patchset:
1. https://codereview.chromium.org/1209953002/
2. https://codereview.chromium.org/1209363003/
3. https://codereview.chromium.org/1208193003/ <-- this patch

BUG=505445

Review URL: https://codereview.chromium.org/1208193003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@198070 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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