Add notifiers for when the audio session will be activated/deactivated, did activate/deactivate and failed to activate/deactivate.

Bug: webrtc:9191
Change-Id: I68a71701dd4c3660331080495b5be4408493aa86
Reviewed-on: https://webrtc-review.googlesource.com/72262
Commit-Queue: JT Teh <jtteh@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23028}

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

@@ -84,6 +84,20 @@ RTC_EXPORT
 - (void)audioSession:(RTCAudioSession *)audioSession
     didDetectPlayoutGlitch:(int64_t)totalNumberOfGlitches;
 
+/** Called when the audio session is about to change the active state.
+ */
+- (void)audioSession:(RTCAudioSession *)audioSession willSetActive:(BOOL)active;
+
+/** Called after the audio session sucessfully changed the active state.
+ */
+- (void)audioSession:(RTCAudioSession *)audioSession didSetActive:(BOOL)active;
+
+/** Called after the audio session failed to change the active state.
+ */
+- (void)audioSession:(RTCAudioSession *)audioSession
+    failedToSetActive:(BOOL)active
+                error:(NSError *)error;
+
 @end
 
 /** This is a protocol used to inform RTCAudioSession when the audio session