Don't depend on WebRTC's default metrics (use webrtc_overrides's one).

In Chromium, WebRTC's metrics implementation is never used. This is
the only use case of them and it is probably possible to use the
Chromium implementation provided by //third_party/webrtc_overrides.

This CL moves metrics specific symbols out of the build target
//third_party/webrtc_overrides:init_webrtc in order to avoid to compile
too much in cases where metrics is the only thing needed.

Bug: webrtc:9631

Change-Id: Id6e07a33eb2df95debee35297d99551224d2ba6e
Reviewed-on: https://chromium-review.googlesource.com/1192923
Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>
Reviewed-by: Tommi <tommi@chromium.org>
Reviewed-by: Gary Kacmarcik <garykac@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589321}
4 files changed