Bug 967970 - Disable 'Saved Application State' in MacOS via plist property. r=smichaud

browser/app/macbuild/Contents/Info.plist.in

@@ -218,5 +218,7 @@
   <true/>
   <key>NSPrincipalClass</key>
   <string>GeckoNSApplication</string>
+  <key>NSDisablePersistence</key>
+  <true/>
 </dict>
 </plist>