Move stats gathering out of LibWebRTCMediaEndpoint

Move stats gathering out of LibWebRTCMediaEndpoint
https://bugs.webkit.org/show_bug.cgi?id=189180

Reviewed by Alejandro G. Castro.

Move stats gathering in LibWebRTCStatsCollector.
Make sure that the lambda given to the collector is always called and destroyed from the main thread.
This allows capturing the promise here instead of storing it into the peer connection backend.
No change of behavior.

* Modules/mediastream/libwebrtc/LibWebRTCMediaEndpoint.cpp:
(WebCore::LibWebRTCMediaEndpoint::getStats):
* Modules/mediastream/libwebrtc/LibWebRTCMediaEndpoint.h:
* Modules/mediastream/libwebrtc/LibWebRTCPeerConnectionBackend.cpp:
(WebCore::LibWebRTCPeerConnectionBackend::getStats):
(WebCore::LibWebRTCPeerConnectionBackend::doStop):
* Modules/mediastream/libwebrtc/LibWebRTCPeerConnectionBackend.h:
* Modules/mediastream/libwebrtc/LibWebRTCStatsCollector.cpp: Added.
(WebCore::LibWebRTCStatsCollector::LibWebRTCStatsCollector):
(WebCore::LibWebRTCStatsCollector::~LibWebRTCStatsCollector):
(WebCore::fromStdString):
(WebCore::fillRTCStats):
(WebCore::fillRTCRTPStreamStats):
(WebCore::fillInboundRTPStreamStats):
(WebCore::fillOutboundRTPStreamStats):
(WebCore::fillRTCMediaStreamTrackStats):
(WebCore::fillRTCDataChannelStats):
(WebCore::iceCandidatePairState):
(WebCore::fillRTCIceCandidatePairStats):
(WebCore::fillRTCCertificateStats):
(WebCore::LibWebRTCStatsCollector::OnStatsDelivered):
* Modules/mediastream/libwebrtc/LibWebRTCStatsCollector.h: Added.
(WebCore::LibWebRTCStatsCollector::create):
* WebCore.xcodeproj/project.pbxproj:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@235576 268f45cc-cd09-0410-ab3c-d52691b4dbfc