Added API to Objective-C PeerConnectionFactoryOptions to enable GCM Ciphers.

This changeset adds the ability for API users to enable or disable GCM Cipher
suites from objective-c.

Bug: chromium:713701
Change-Id: I0ac7b60f55dd56bebbcfb315a542ef4843099802
Reviewed-on: https://webrtc-review.googlesource.com/89263
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Commit-Queue: Benjamin Wright <benwright@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24028}

sdk/objc/Framework/Headers/WebRTC/RTCPeerConnectionFactoryOptions.h

@@ -33,6 +33,8 @@ RTC_EXPORT
 
 @property(nonatomic, assign) BOOL enableAes128Sha1_32CryptoCipher;
 
+@property(nonatomic, assign) BOOL enableGcmCryptoSuites;
+
 - (instancetype)init NS_DESIGNATED_INITIALIZER;
 
 @end