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This adds a CronetMetricsDelegate interface, as well as addMetricsDelegate
and removeMetricsDelegate methods, which are used to register/unregister
metrics callbacks.

Currently the callback is never actually called; this is simply defining the
interface and registration methods.

Bug: 702796
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I0a3933bf623265d675e3a10ef5ed25149c89e101
Reviewed-on: https://chromium-review.googlesource.com/748819
Commit-Queue: Lily Houghton <lilyhoughton@chromium.org>
Reviewed-by: Andrei Kapishnikov <kapishnikov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515456}