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}

examples/objc/AppRTCMobile/ARDSettingsModel.m

@@ -169,20 +169,12 @@ NS_ASSUME_NONNULL_BEGIN
 
 - (void)registerStoreDefaults {
   NSString *defaultVideoResolutionSetting = [self defaultVideoResolutionSetting];
-  BOOL audioOnly = (defaultVideoResolutionSetting.length == 0);
-
-// The iOS simulator doesn't provide any sort of camera capture
-// support or emulation (http://goo.gl/rHAnC1) so don't bother
-// trying to open a local stream.
-#if TARGET_IPHONE_SIMULATOR
-  audioOnly = YES;
-#endif
 
   NSData *codecData = [NSKeyedArchiver archivedDataWithRootObject:[self defaultVideoCodecSetting]];
   [ARDSettingsStore setDefaultsForVideoResolution:[self defaultVideoResolutionSetting]
                                        videoCodec:codecData
                                           bitrate:nil
-                                        audioOnly:audioOnly
+                                        audioOnly:NO
                                     createAecDump:NO
                                useLevelController:NO
                              useManualAudioConfig:YES];