Stop hard-coding default IDs for RTP extensions

Hard-coding default values forces IDs over 14 to be used even
when we offer less than 15 different extensions.

Note that the code relies on MergeRtpHdrExts for making sure
that extension IDs are kept consistent and non-colliding between
different streams (audio/video).

Bug: webrtc:10288
Change-Id: I3e59f7ddc8ca43cea91084a6b7f36df70fb6be4a
Reviewed-on: https://webrtc-review.googlesource.com/c/121646
Commit-Queue: Elad Alon <eladalon@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26622}
diff --git a/video/BUILD.gn b/video/BUILD.gn
index 67d9d4b..cb02610 100644
--- a/video/BUILD.gn
+++ b/video/BUILD.gn
@@ -553,6 +553,7 @@
       "../modules/video_coding:webrtc_multiplex",
       "../modules/video_coding:webrtc_vp8",
       "../modules/video_coding:webrtc_vp9",
+      "../rtc_base",
       "../rtc_base:checks",
       "../rtc_base:rate_limiter",
       "../rtc_base:rtc_base_approved",