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This patch does three things:

1. Implements the Blink platform interface WebCredentialManagerClient by
   adding a CredentialManagerClient class to the password manager component.

2. Wires that implementation up to Blink's platform layer by creating an
   instance of the new client, held in ChromeContentRendererClient, and
   setting it as each new RenderView's client.

3. Stubs out IPCs for the renderer to pass messages up to the browser in
   order to do the heavy lifting of actually answering Blink's requests by
   generating and delivering Credential objects.                                                                                                             

BUG=400674
TBR=tkent@chromium.org

Review URL: https://codereview.chromium.org/464883002

Cr-Commit-Position: refs/heads/master@{#291390}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@291390 0039d316-1c4b-4281-b951-d872f2087c98