Clean up dependencies of peerconnection_unittest.

There were a number of unused includes and undeclared
dependencies. I removed the includes that were causing
problems and added dependencies for the includes that
turned out to be needed.

Bug: webrtc:7239,webrtc:6828
Change-Id: I5b57f9b8411d969e96eaa46fb49101b7b7c32284
Reviewed-on: https://webrtc-review.googlesource.com/1185
Commit-Queue: Patrik Höglund <phoglund@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#19858}
diff --git a/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index 31848df..7a76b7e 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -777,6 +777,7 @@
     "nattypes.h",
     "proxyserver.cc",
     "proxyserver.h",
+    "refcount.h",
     "sha1.cc",
     "sha1.h",
     "sha1digest.cc",