Implements the RTCPeerConnection.getStats(MediaStreamTrack) version[1]
of the stats selection algorithm behind flag. This is the equivalent of
doing RTCRtpSender.getStats() or RTCRtpReceiver.getStats() for the
track's sender or receiver. This is tested in external/wpt/.

Due to limitations of the generated V8 bindings it is not possible to
express all versions of RTCPeerConnection.getStats() at the same time
in IDL. Thus this CL introduces a version of getStats() taking
"optional any callbacksOrSelector" as argument, with custom binding
logic performed in RTCPeerConnection.cpp. A fast/peerconnection/
LayoutTest is added to test all combinations.

[1] https://w3c.github.io/webrtc-pc/#dom-rtcpeerconnection-getstats

Bug: 680172
Change-Id: I8c64fc64c708d266c926dfa3eb3587c4cbb31210
Reviewed-on: https://chromium-review.googlesource.com/978128
Commit-Queue: Henrik Boström <hbos@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548149}