Clean up unused ContentBrowserClient methods that access cookie settings.

This CL deletes the following two methods in ContentBrowserClient that were
used to retrieve cookie settings on behalf of features that no longer exist:

ContentBrowserClient::AllowSaveLocalState
ContentBrowserClient::AllowWebRTCIdentityCache

See the bug for details.

BUG=724442,webrtc:5708,493212

Review-Url: https://codereview.chromium.org/2888373003
Cr-Commit-Position: refs/heads/master@{#473656}
4 files changed