[iOS] Fix incorrectly oriented frames when rapidly switching between cameras.

webrtc/sdk/objc/Framework/Classes/Video/AVCaptureSession+Device.mm

+/*
+ *  Copyright 2017 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.
+ */
+
+#import "AVCaptureSession+Device.h"
+
+BOOL CFStringContainsString(CFStringRef theString, CFStringRef stringToFind) {
+  return CFStringFindWithOptions(theString,
+                                 stringToFind,
+                                 CFRangeMake(0, CFStringGetLength(theString)),
+                                 kCFCompareCaseInsensitive,
+                                 nil);
+}
+
+@implementation AVCaptureSession (Device)
+
++ (AVCaptureDevicePosition)devicePositionForSampleBuffer:(CMSampleBufferRef)sampleBuffer {
+  // Check the image's EXIF for the actual camera the image came.
+  AVCaptureDevicePosition cameraPosition = AVCaptureDevicePositionUnspecified;
+  CFDictionaryRef attachments = CMCopyDictionaryOfAttachments(
+      kCFAllocatorDefault, sampleBuffer, kCMAttachmentMode_ShouldPropagate);
+  if (attachments) {
+    int size = CFDictionaryGetCount(attachments);
+    if (size > 0) {
+      CFDictionaryRef cfExifDictVal = nil;
+      if (CFDictionaryGetValueIfPresent(
+              attachments, (const void *)CFSTR("{Exif}"), (const void **)&cfExifDictVal)) {
+        CFStringRef cfLensModelStrVal;
+        if (CFDictionaryGetValueIfPresent(cfExifDictVal,
+                                          (const void *)CFSTR("LensModel"),
+                                          (const void **)&cfLensModelStrVal)) {
+          if (CFStringContainsString(cfLensModelStrVal, CFSTR("front"))) {
+            cameraPosition = AVCaptureDevicePositionFront;
+          } else if (CFStringContainsString(cfLensModelStrVal, CFSTR("back"))) {
+            cameraPosition = AVCaptureDevicePositionBack;
+          }
+        }
+      }
+    }
+    CFRelease(attachments);
+  }
+  return cameraPosition;
+}
+
+@end