Allow external initialization of libsrtp.

Bug: webrtc:11205
Change-Id: I906651e3afc5c50977ff567f13a44e5087604028
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/161952
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30074}

pc/srtp_session.cc

@@ -366,6 +366,11 @@ bool SrtpSession::UpdateKey(int type,
 ABSL_CONST_INIT int g_libsrtp_usage_count = 0;
 ABSL_CONST_INIT rtc::GlobalLock g_libsrtp_lock;
 
+void ProhibitLibsrtpInitialization() {
+  rtc::GlobalLockScope ls(&g_libsrtp_lock);
+  ++g_libsrtp_usage_count;
+}
+
 // static
 bool SrtpSession::IncrementLibsrtpUsageCountAndMaybeInit() {
   rtc::GlobalLockScope ls(&g_libsrtp_lock);

pc/srtp_session.h

@@ -22,6 +22,11 @@ struct srtp_ctx_t_;
 
 namespace cricket {
 
+// Prohibits webrtc from initializing libsrtp. This can be used if libsrtp is
+// initialized by another library or explicitly. Note that this must be called
+// before creating an SRTP session with WebRTC.
+void ProhibitLibsrtpInitialization();
+
 // Class that wraps a libSRTP session.
 class SrtpSession {
  public: