Fix //components/os_crypt to build for iOS with gn.

components/os_crypt/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 source_set("os_crypt") {
   sources = [
     "ie7_password_win.cc",
     "ie7_password_win.h",
     "keychain_password_mac.h",
     "keychain_password_mac.mm",
     "os_crypt.h",
     "os_crypt_mac.mm",
     "os_crypt_posix.cc",
     "os_crypt_switches.cc",
     "os_crypt_switches.h",
     "os_crypt_win.cc",
   ]
 
   deps = [
     "//base",
     "//crypto",
 
     # TODO(tfarina): Remove this dep when http://crbug.com/363749 is fixed.
     "//crypto:platform",
   ]
 
-  if (is_mac) {
+  if (is_mac || is_ios) {
     sources -= [ "os_crypt_posix.cc" ]
   }
 
   if (is_ios) {
+    set_sources_assignment_filter([])
     sources += [
       "keychain_password_mac.mm",
       "os_crypt_mac.mm",
     ]
+    set_sources_assignment_filter(sources_assignment_filter)
   }
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "ie7_password_win_unittest.cc",
     "keychain_password_mac_unittest.mm",
     "os_crypt_unittest.cc",
   ]
   deps = [
     ":os_crypt",
     "//base",
     "//crypto",
     "//testing/gtest",
   ]
 }