Start consolidating management/querying of stats on the Call thread.

Call is instantiated on what we traditionally call the 'worker thread'
in PeerConnection terms. Call statistics are however gathered, processed
and reported in a number of different ways, which results in a lot of
locking, which is also unpredictable due to the those actions themselves
contending with other parts of the system.

Designating the worker thread as the general owner of the stats, helps
us keeps things regular and avoids loading unrelated task queues/threads
with reporting things like histograms or locking up due to a call to
GetStats().

This is a reland of remaining changes from https://webrtc-review.googlesource.com/c/src/+/172847:
This applies the changes from the above CL to the forked files and
switches call.cc over to using the forked implementation.

Bug: webrtc:11489
Change-Id: I93ad560500806ddd0e6df1448b1bcf5a1aae7583
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/174000
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Magnus Flodman <mflodman@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31186}

modules/video_coding/generic_decoder.cc

@@ -57,6 +57,8 @@ VCMReceiveCallback* VCMDecodedFrameCallback::UserReceiveCallback() {
 }
 
 int32_t VCMDecodedFrameCallback::Decoded(VideoFrame& decodedImage) {
+  // This function may be called on the decode TaskQueue, but may also be called
+  // on an OS provided queue such as on iOS (see e.g. b/153465112).
   return Decoded(decodedImage, -1);
 }