Reland "Add file capturer to AppRTCMobile on simulator."

This is a reland of 5adcd198752b651f7b7e9199a91f9b873b7d7237
Original change's description:
> Add file capturer to AppRTCMobile on simulator.
> 
> To achieve this, the CL does the following
> - Adds sample mp4 video
> - Refactors the existing RTCFileVideoCapturer to achieve continious
> capture and adds tests.
> 
> Bug: webrtc:8406
> Change-Id: Ibc0891176c58ec9053b42e340d2113036e7199ec
> Reviewed-on: https://webrtc-review.googlesource.com/12180
> Reviewed-by: Anders Carlsson <andersc@webrtc.org>
> Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
> Commit-Queue: Daniela Jovanoska Petrenko <denicija@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#20598}

Bug: webrtc:8406
Change-Id: I93be89b86e342a9a8195e19ebaf4aef1410d2c20
Reviewed-on: https://webrtc-review.googlesource.com/23200
Reviewed-by: Daniela Jovanoska Petrenko <denicija@webrtc.org>
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Commit-Queue: Daniela Jovanoska Petrenko <denicija@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20870}
diff --git a/examples/objc/AppRTCMobile/ARDAppClient.h b/examples/objc/AppRTCMobile/ARDAppClient.h
index 8c27b34..5054c28 100644
--- a/examples/objc/AppRTCMobile/ARDAppClient.h
+++ b/examples/objc/AppRTCMobile/ARDAppClient.h
@@ -9,8 +9,6 @@
  */
 
 #import <Foundation/Foundation.h>
-
-#import "WebRTC/RTCCameraVideoCapturer.h"
 #import "WebRTC/RTCPeerConnection.h"
 #import "WebRTC/RTCVideoTrack.h"
 
@@ -26,6 +24,8 @@
 @class ARDAppClient;
 @class ARDSettingsModel;
 @class RTCMediaConstraints;
+@class RTCCameraVideoCapturer;
+@class RTCFileVideoCapturer;
 
 // The delegate is informed of pertinent events and will be called on the
 // main queue.
@@ -52,6 +52,10 @@
 - (void)appClient:(ARDAppClient *)client
       didGetStats:(NSArray *)stats;
 
+@optional
+- (void)appClient:(ARDAppClient *)client
+didCreateLocalFileCapturer:(RTCFileVideoCapturer *)fileCapturer;
+
 @end
 
 // Handles connections to the AppRTC server for a given room. Methods on this