Add dummy V8 build args to prevent for the iOS build.

ios/BUILD.gn

 # Copyright 2016 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.
 
 assert(!is_component_build, "component build is unsupported on iOS")
 
 declare_args() {
   # Control whether cronet is build (this is usually set by the script
   # components/cronet/tools/cr_cronet.py as cronet requires specific
   # gn args to build correctly).
   is_cronet_build = false
+
+  # TODO(brettw) bug 684096: Remove these. These are to prevent unused
+  # override warnings on iOS from the setting of the default_args in the
+  # toplevel //.gn file. Since iOS doesn't compile V8, GN complains that
+  # the overrides are never used.
+  v8_extra_library_files = []
+  v8_experimental_extra_library_files = []
+  v8_enable_gdbjit = false
+  v8_imminent_deprecation_warnings = false
 }
 
+# Prevent warnings for unused build args above.
+assert(v8_extra_library_files != 0)
+assert(v8_experimental_extra_library_files != 0)
+assert(!v8_enable_gdbjit)
+assert(!v8_imminent_deprecation_warnings)
+
 # This list all targets that needs to be build as part of "gn_all" on iOS.
 # This list should generally only include executables, but since some code
 # has not yet been upstreamed it will sometimes also include targets that
 # are not used upstream to ensure they are not broken inadvertently.
 group("all") {
   testonly = true
   if (is_cronet_build) {
     deps = [
       "//components/cronet/ios:cronet_package",
       "//ios/crnet:crnet_framework",
@@ skipping 17 matching lines @@
       "//ios/chrome/test/earl_grey:all_tests",
       "//ios/clean/chrome/test:all_tests",
       "//ios/net:all_tests",
       "//ios/showcase:all_tests",
       "//ios/testing:all_tests",
       "//ios/web:all_tests",
       "//ios/web/shell/test:all_tests",
     ]
   }
 }