Move webrtc::CreatePeerConnectionFactory definition next to decl.

This CL moves webrtc::CreatePeerConnectionFactory definitions out of
pc:create_pc_factory and merges it with its declaration in the api/
directory.

In order to avoid circular dependencies a new build target is created:
* api:create_peerconnection_factory

Bug: webrtc:9862
Change-Id: Ie215c94460cba026f5bf7d11c9a5aa03792064af
Reviewed-on: https://webrtc-review.googlesource.com/c/111186
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25744}
diff --git a/pc/peerconnection_rampup_tests.cc b/pc/peerconnection_rampup_tests.cc
index 850212b..74f8018 100644
--- a/pc/peerconnection_rampup_tests.cc
+++ b/pc/peerconnection_rampup_tests.cc
@@ -10,6 +10,7 @@
 
 #include "api/audio_codecs/builtin_audio_decoder_factory.h"
 #include "api/audio_codecs/builtin_audio_encoder_factory.h"
+#include "api/create_peerconnection_factory.h"
 #include "api/stats/rtcstats_objects.h"
 #include "api/video_codecs/builtin_video_decoder_factory.h"
 #include "api/video_codecs/builtin_video_encoder_factory.h"