Add usage description strings to Info.plist

webrtc/examples/objc/AppRTCMobile/ios/Info.plist

 <?xml version="1.0" encoding="UTF-8"?>
 <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
 <plist version="1.0">
 <dict>
   <key>BuildMachineOSBuild</key>
   <string>12E55</string>
   <key>CFBundleDevelopmentRegion</key>
   <string>en</string>
   <key>CFBundleDisplayName</key>
   <string>AppRTCMobile</string>
@@ skipping 43 matching lines @@
   </array>
   <key>UIAppFonts</key>
   <array>
     <string>Roboto-Regular.ttf</string>
   </array>
   <key>UIBackgroundModes</key>
   <array>
     <string>audio</string>
     <string>voip</string>
   </array>
+  <key>NSCameraUsageDescription</key>
+  <string>AppRTCDemo needs access to the camera</string>

[tkchin_webrtc, 2016/09/27 13:41:07]

Consider not using the app name here.
e.g. Camera access needed for video
Microphone access needed for audio

[kthelgason, 2016/09/27 13:55:33]

Acknowledged.

+  <key>NSMicrophoneUsageDescription</key>
+  <string>AppRTCDemo needs access to the microphone</string>
   <key>UILaunchImages</key>
   <array>
     <dict>
       <key>UILaunchImageMinimumOSVersion</key>
       <string>7.0</string>
       <key>UILaunchImageName</key>
       <string>iPhone5</string>
       <key>UILaunchImageOrientation</key>
       <string>Portrait</string>
       <key>UILaunchImageSize</key>
@@ skipping 15 matching lines @@
       <key>UILaunchImageName</key>
       <string>iPhone6p</string>
       <key>UILaunchImageOrientation</key>
       <string>Portrait</string>
       <key>UILaunchImageSize</key>
       <string>{414, 736}</string>
     </dict>
   </array>
 </dict>
 </plist>