Put iOS H264 High profile under a field trial

H264 High profile is causing problems for some downstream apps. Therefore,
put it behind a field trial instead.

BUG=webrtc:6337

Review-Url: https://codereview.webrtc.org/2580963004
Cr-Commit-Position: refs/heads/master@{#15663}
diff --git a/webrtc/sdk/objc/Framework/Classes/RTCFieldTrials.mm b/webrtc/sdk/objc/Framework/Classes/RTCFieldTrials.mm
index 224f7fd..89cf3c8 100644
--- a/webrtc/sdk/objc/Framework/Classes/RTCFieldTrials.mm
+++ b/webrtc/sdk/objc/Framework/Classes/RTCFieldTrials.mm
@@ -20,6 +20,7 @@
 NSString * const kRTCFieldTrialFlexFec03Key = @"WebRTC-FlexFEC-03";
 NSString * const kRTCFieldTrialImprovedBitrateEstimateKey = @"WebRTC-ImprovedBitrateEstimate";
 NSString * const kRTCFieldTrialTrendlineFilterKey = @"WebRTC-BweTrendlineFilter";
+NSString * const kRTCFieldTrialH264HighProfileKey = @"WebRTC-H264HighProfile";
 NSString * const kRTCFieldTrialEnabledValue = @"Enabled";
 
 static std::unique_ptr<char[]> gFieldTrialInitString;