Fix partial availability warnings on Mac AppRTCMobile

webrtc/examples/objc/AppRTCMobile/mac/APPRTCViewController.m

 /*
  *  Copyright 2014 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 224 matching lines @@
   [self addSubview:_localVideoView];
 }
 
 - (void)setupActionItemsView {
   _actionItemsView = [[NSView alloc] initWithFrame:NSZeroRect];
   [_actionItemsView setTranslatesAutoresizingMaskIntoConstraints:NO];
   [self addSubview:_actionItemsView];
 
   _roomField = [[NSTextField alloc] initWithFrame:NSZeroRect];
   [_roomField setTranslatesAutoresizingMaskIntoConstraints:NO];
-  _roomField.placeholderString = @"Enter AppRTC room id";
+  [[_roomField cell] setPlaceholderString: @"Enter AppRTC room id"];
   [_actionItemsView addSubview:_roomField];
   [_roomField setEditable:YES];
 
   _connectButton = [[NSButton alloc] initWithFrame:NSZeroRect];
   [_connectButton setTranslatesAutoresizingMaskIntoConstraints:NO];
   _connectButton.title = @"Start call";
   _connectButton.bezelStyle = NSRoundedBezelStyle;
   _connectButton.target = self;
   _connectButton.action = @selector(startCall:);
   [_actionItemsView addSubview:_connectButton];
@@ skipping 142 matching lines @@
   [self.mainView.remoteVideoView renderFrame:nil];
   [self.mainView.localVideoView renderFrame:nil];
 }
 
 - (void)disconnect {
   [self resetUI];
   [_client disconnect];
 }
 
 @end