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This CL implements experimental feature to skip 'Sign In' button
in OptIn flow. Currently it is implemented as 'sandbox' on server
side and this is available only for internal IP addresses. It is
optional and can be activated in chrome://flags.

BUG=b/28987931
TEST=Manually on device in both modes.

Review-Url: https://codereview.chromium.org/2320813002
Cr-Commit-Position: refs/heads/master@{#417180}