Update ios-simulator-cronet to use the 9.3 SDK

ios/build/bots/chromium.mac/ios-simulator-cronet.json

 {
   "owners": [
     "smut"
   ],
   "comments": [
     "Cronet tests for 32- and 64-bit iOS 9.0 simulators.",
     "Tests run on iPhone 5s (64-bit) and iPad Retina (32-bit)."
   ],
-  "xcode version": "7.0",
+  "xcode version": "7.3",
   "GYP_DEFINES": [
     "OS=ios",
     "chromium_ios_signing=0",
     "component=static_library",
     "enable_websockets=false",
     "disable_brotli_filter=true",
     "disable_file_support=true",
     "disable_ftp_support=true",
     "gomadir=$(goma_dir)",
     "target_subarch=both",
     "use_goma=1",
     "use_platform_icu_alternatives=true"
   ],
   "mb_type": "gyp",
   "compiler": "ninja",
   "configuration": "Debug",
-  "sdk": "iphonesimulator9.0",
+  "sdk": "iphonesimulator9.3",
   "tests": [
     {
       "app": "cronet_test",
       "device type": "iPhone 5s",
-      "os": "9.0"
+      "os": "9.3"
     },
     {
       "app": "cronet_test",
       "device type": "iPad Retina",
-      "os": "9.0"
+      "os": "9.3"
     }
   ]
 }