This is a resubmission of https://codereview.webrtc.org/2047513002/

Original description:
Add proper lifetime of encoder-specific settings.

Permits passing VideoEncoderConfig between threads and not worry about
the lifetime of an underlying void pointer. Also adds type safety to
unpacking of codec-specific settings.

These settings are not yet propagating to VideoEncoder interfaces, but
the aim is to get rid of webrtc::VideoCodec for VideoEncoder.

BUG=webrtc:3424
R=perkj@webrtc.org, pbos@webrtc.org
TBR=mflodman@webrtc.org

Review-Url: https://codereview.webrtc.org/2347843002
Cr-Commit-Position: refs/heads/master@{#14396}

webrtc/modules/audio_coding/BUILD.gn

@@ -1317,6 +1317,7 @@ if (rtc_include_tests) {
     testonly = true
     deps = [
       "../..:webrtc_common",
+      "../../base:rtc_base_approved",
       "//testing/gtest",
     ]
     sources = [