Enable experimental WebRTC SVC feature

Spec: https://w3c.github.io/webrtc-svc/
Intent to implement: https://groups.google.com/a/chromium.org/forum/?utm_medium=email&utm_source=footer#!msg/blink-dev/--ZpYlTo6LE/h0RzUIWkAAAJ
Chrome dashboard: https://www.chromestatus.com/feature/5769626174619648

Bug: 986069
Change-Id: Ia8f2cb90ec26eb83528f0f45acbc5ccbdf2ad030
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1868989Reviewed-by: Florent Castelli <orphis@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707916}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -1423,6 +1423,7 @@
     // Spec: https://w3c.github.io/webrtc-svc/
     {
       name: "RTCSvcScalabilityMode",
+      status: "experimental",
     },
     // Enables the use of |RTCConfiguration::sdpSemantics| to override the
     // default SDP semantics at RTCPeerConnection construction.

third_party/blink/web_tests/external/wpt/webrtc-svc/RTCRtpParameters-scalability-expected.txt

-This is a testharness.js-based test.
-FAIL Setting scalabilityMode to a legal value should be accepted assert_equals: expected (string) "L1T3" but got (undefined) undefined
-FAIL Sender capabilities should include at least some scalability modes assert_true: expected true got false
-Harness: the test ran to completion.
-