Support encrypted RTP extensions (RFC 6904)

Can be enabled by setting "enable_encrypted_rtp_header_extensions" in
"crypto_options" of "PeerConnectionFactoryInterface::Options" and will
only be used if both peers support it.

BUG=webrtc:3411

Review-Url: https://codereview.webrtc.org/2761143002
Cr-Commit-Position: refs/heads/master@{#18842}
diff --git a/webrtc/media/BUILD.gn b/webrtc/media/BUILD.gn
index 304383b..11a1aa3 100644
--- a/webrtc/media/BUILD.gn
+++ b/webrtc/media/BUILD.gn
@@ -327,6 +327,7 @@
     sources = [
       "base/fakemediaengine.h",
       "base/fakenetworkinterface.h",
+      "base/fakertp.cc",
       "base/fakertp.h",
       "base/fakevideocapturer.h",
       "base/fakevideorenderer.h",