Add support for GCM cipher suites from RFC 7714.

GCM cipher suites are optional (disabled by default) and can be enabled
through "PeerConnectionFactoryInterface::Options".

If compiled with Chromium (i.e. "ENABLE_EXTERNAL_AUTH" is defined), no
GCM ciphers can be used yet (see https://crbug.com/628400).

BUG=webrtc:5222, 628400

Review-Url: https://codereview.webrtc.org/1528843005
Cr-Commit-Position: refs/heads/master@{#13635}
diff --git a/webrtc/api/peerconnectionfactory.h b/webrtc/api/peerconnectionfactory.h
index c209fdb..377ad73 100644
--- a/webrtc/api/peerconnectionfactory.h
+++ b/webrtc/api/peerconnectionfactory.h
@@ -31,9 +31,7 @@
 
 class PeerConnectionFactory : public PeerConnectionFactoryInterface {
  public:
-  void SetOptions(const Options& options) override {
-    options_ = options;
-  }
+  void SetOptions(const Options& options) override;
 
   // Deprecated, use version without constraints.
   rtc::scoped_refptr<PeerConnectionInterface> CreatePeerConnection(