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This adds a new AddServiceOverride method to ServiceRegistryImpl.
The purpose of this is to allow testing code to intercept remote
service connections and opaquely provide a mock implementation of
any service.

The new interface is also added to the ServiceRegistry's JS
bindings.

Review URL: https://codereview.chromium.org/1470153002

Cr-Commit-Position: refs/heads/master@{#371736}