Correct iOS build for latest Xcode beta.

ios/net/empty_nsurlcache.mm

Index: ios/net/empty_nsurlcache.mm
diff --git a/third_party/mojo/src/mojo/public/go/system/system_nacl.go b/ios/net/empty_nsurlcache.mm
similarity index 50%
copy from third_party/mojo/src/mojo/public/go/system/system_nacl.go
copy to ios/net/empty_nsurlcache.mm
index 8bf001c1e66a4267b1b760be4e91b6b8176e1dd9..621f4b1b12f36b2f08ce453f4fb61aa782048925 100644
--- a/third_party/mojo/src/mojo/public/go/system/system_nacl.go
+++ b/ios/net/empty_nsurlcache.mm
@@ -2,11 +2,13 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-package system
+#import "ios/net/empty_nsurlcache.h"
 
-import "runtime/nacl"
+@implementation EmptyNSURLCache
 
-// NaCl uses the runtime's Mojo IRT based system implementation.
-func init() {
-	sysImpl = &nacl.MojoNaClSystem{}
+- (NSCachedURLResponse*)cachedResponseForRequest:
+        (NSURLRequest*)request {
+  return nil;
 }
+
+@end