Report available instead of encoding bitrate to VideoEncoderSelector.

The encoding bitrate might be limited depending on the current encoder.

Bug: webrtc:11341
Change-Id: I734fce12734b1e703e7948847cdb1365c08a137b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/169123
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Commit-Queue: Mirta Dvornicic <mirtad@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30619}

api/video_codecs/video_encoder_factory.h

@@ -49,9 +49,9 @@ class VideoEncoderFactory {
     // used.
     virtual void OnCurrentEncoder(const SdpVideoFormat& format) = 0;
 
-    // Called every time the encoding bitrate is updated. Should return a
+    // Called every time the available bitrate is updated. Should return a
     // non-empty if an encoder switch should be performed.
-    virtual absl::optional<SdpVideoFormat> OnEncodingBitrate(
+    virtual absl::optional<SdpVideoFormat> OnAvailableBitrate(
         const DataRate& rate) = 0;
 
     // Called if the currently used encoder reports itself as broken. Should