Revert "Remove precompiled header for AppRTCMobile."

This reverts commit 3ed32accc2efab456ec4eedf9df4cef1df6b357d.

Reason for revert: Compilation failure.

Original change's description:
> Remove precompiled header for AppRTCMobile.
> 
> Bug: None
> Change-Id: Ia46fc3a237a882acef5218ef22c283fb9c379e44
> Reviewed-on: https://webrtc-review.googlesource.com/3340
> Reviewed-by: Anders Carlsson <andersc@webrtc.org>
> Commit-Queue: Kári Helgason <kthelgason@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#19948}

TBR=andersc@webrtc.org,kthelgason@webrtc.org

Change-Id: Id6fc5d4978315be13da7ef03438c0804fa19c4a1
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: None
Reviewed-on: https://webrtc-review.googlesource.com/3500
Reviewed-by: Charu Jain <charujain@webrtc.org>
Commit-Queue: Charu Jain <charujain@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#19952}

examples/objc/AppRTCMobile/ios/AppRTCMobile-Prefix.pch

@@ -0,0 +1,23 @@
+/*
+ *  Copyright 2013 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.
+ */
+
+//
+// Prefix header for all source files of the 'AppRTCMobile' target in the
+// 'AppRTCMobile' project
+//
+
+#import <Availability.h>
+
+#if __IPHONE_OS_VERSION_MIN_REQUIRED < __IPHONE_6_0
+#warning "This project uses features only available in iOS SDK 6.0 and later."
+#endif
+
+#import <Foundation/Foundation.h>
+#import <UIKit/UIKit.h>