Activate/deactivate VP9 spatial layers.

* Stop encoding spatial layers S(n >= N) if application deactivates
spatial layer N by setting RTCRtpEncodingParameters.active = false.

* Move calculation of padding bitrate to SvcRateAllocator class.

* Pad up to minimum required bitrate of base layer if ALR probing is
enabled.

Bug: webrtc:9350
Change-Id: I398284c943d43348def535c83263fc234c9767fa
Reviewed-on: https://webrtc-review.googlesource.com/c/113240
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25945}
diff --git a/video/BUILD.gn b/video/BUILD.gn
index af08bb7..a554213 100644
--- a/video/BUILD.gn
+++ b/video/BUILD.gn
@@ -190,6 +190,7 @@
     "../common_video:common_video",
     "../modules/video_coding",
     "../modules/video_coding:video_coding_utility",
+    "../modules/video_coding:webrtc_vp9_helpers",
     "../rtc_base:checks",
     "../rtc_base:criticalsection",
     "../rtc_base:logging",