Revert "Move stats ID generation from SSRC to local ID"

This reverts commit e357a4dd4e3b015f8281813f246de793589bd537.

Reason for revert: Looks like it's breaking some downstream projects.

Original change's description:
> Move stats ID generation from SSRC to local ID
> 
> This generates stats IDs for Track stats (which
> represents stats on the attachment of a track to
> a PeerConnection) from being SSRC-based to being
> based on an ID that is allocated when connecting the
> track to the PC.
> 
> This is a prerequisite to generating stats before
> the PeerConnection is connected.
> 
> Bug: webrtc:8673
> Change-Id: I82f6e521646b0c92b3af4dffb2cdee75e6dc10d4
> Reviewed-on: https://webrtc-review.googlesource.com/38360
> Commit-Queue: Harald Alvestrand <hta@webrtc.org>
> Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
> Reviewed-by: Henrik Boström <hbos@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#21582}

TBR=solenberg@webrtc.org,hbos@webrtc.org,hta@webrtc.org

Change-Id: I621c10236c02be01d82f4660168f0323b85e24af
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:8673
Reviewed-on: https://webrtc-review.googlesource.com/38681
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21586}

api/test/mock_rtpreceiver.h

@@ -29,7 +29,6 @@ class MockRtpReceiver : public rtc::RefCountedObject<RtpReceiverInterface> {
   MOCK_METHOD1(SetParameters, bool(const RtpParameters&));
   MOCK_METHOD1(SetObserver, void(RtpReceiverObserverInterface*));
   MOCK_CONST_METHOD0(GetSources, std::vector<RtpSource>());
-  MOCK_CONST_METHOD0(AttachmentId, int());
 };
 
 }  // namespace webrtc