Move sessiondescription.h/cc from p2p/base to pc/

SDP is a detail of PeerConnection and is not used by anything in p2p, so
it belongs in the pc/ directory. This also allows
MediaContentDescription to be inlined in the future.

Bug: webrtc:8620
Change-Id: I38b65ede9942e29eb15035ab29f2be988da1e5ce
Reviewed-on: https://webrtc-review.googlesource.com/33781
Reviewed-by: Niklas Enbom <niklas.enbom@webrtc.org>
Reviewed-by: Peter Thatcher <pthatcher@webrtc.org>
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21376}

pc/peerconnection_integrationtest.cc

diff --git a/pc/peerconnection_integrationtest.cc b/pc/peerconnection_integrationtest.cc
index d301d98..6ae4a92 100644
--- a/pc/peerconnection_integrationtest.cc
+++ b/pc/peerconnection_integrationtest.cc
@@ -33,7 +33,6 @@
 #include "media/engine/fakewebrtcvideoengine.h"
 #include "p2p/base/p2pconstants.h"
 #include "p2p/base/portinterface.h"
-#include "p2p/base/sessiondescription.h"
 #include "p2p/base/teststunserver.h"
 #include "p2p/base/testturncustomizer.h"
 #include "p2p/base/testturnserver.h"
@@ -43,6 +42,7 @@
 #include "pc/mediasession.h"
 #include "pc/peerconnection.h"
 #include "pc/peerconnectionfactory.h"
+#include "pc/sessiondescription.h"
 #include "pc/test/fakeaudiocapturemodule.h"
 #include "pc/test/fakeperiodicvideocapturer.h"
 #include "pc/test/fakertccertificategenerator.h"