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This is a large improvement from the previous duplicated lists of
source files in WebRTC. This moves over to using the GYP/GN targets
in third_party/webrtc instead, which has become possible after landing
https://codereview.webrtc.org/1924663003/ and
https://codereview.webrtc.org/2037983002/.

BUG=webrtc:4256
TESTED=Local builds passing for Linux and Android.

Review-Url: https://codereview.chromium.org/2022833002
Cr-Commit-Position: refs/heads/master@{#408931}