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Use ExtensionsAPIClient to register guests.

BUG=352290

Review URL: https://codereview.chromium.org/444113003

Cr-Commit-Position: refs/heads/master@{#288427}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288427 0039d316-1c4b-4281-b951-d872f2087c98