Adding flag to enable/disable use of SRTP_AES128_CM_SHA1_32 crypto suite.

This flag (added to CryptoOptions) will allow applications to opt-in to
use of this suite, before it's disabled by default later. See bug for
more details.

TBR=magjed@webrtc.org

Bug: webrtc:7670
Change-Id: I800bedd4b26d807b6b7ac66b505d419c3323e454
Reviewed-on: https://webrtc-review.googlesource.com/64390
Commit-Queue: Taylor Brandstetter <deadbeef@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22586}

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

@@ -31,6 +31,8 @@ RTC_EXPORT
 
 @property(nonatomic, assign) BOOL ignoreEthernetNetworkAdapter;
 
+@property(nonatomic, assign) BOOL enableAes128Sha1_32CryptoCipher;
+
 - (instancetype)init NS_DESIGNATED_INITIALIZER;
 
 @end