Add flag to control whether cronet is build on iOS.

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
+}
+
 # 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
-  deps = [
-    "//ios/chrome:ios_chrome_unittests",
-    "//ios/chrome/app",
-    "//ios/chrome/browser",
-    "//ios/chrome/common",
-    "//ios/chrome/common/physical_web",
-    "//ios/chrome/share_extension:packed_resources",
-    "//ios/chrome/today_extension:packed_resources",
-    "//ios/net:ios_net_unittests",
-    "//ios/public/provider/chrome/browser",
-    "//ios/public/provider/web",
-    "//ios/testing:ocmock_support_unittest",
-    "//ios/third_party/earl_grey",
-    "//ios/web:earl_grey_test_support",
-    "//ios/web:ios_web_inttests",
-    "//ios/web:ios_web_unittests",
-    "//ios/web/shell:ios_web_shell",
-    "//ios/web/shell/test:ios_web_shell_test_host",
-  ]
-
-  if (!is_component_build) {
-    deps += [
+  if (is_cronet_build) {
+    deps = [
       "//components/cronet/ios:cronet_package",
       "//ios/crnet:crnet_framework",
       "//ios/crnet/crnet_consumer",
       "//ios/crnet/test:crnet_test",
     ]
+  } else {
+    deps = [
+      "//ios/chrome:ios_chrome_unittests",
+      "//ios/chrome/app",
+      "//ios/chrome/browser",
+      "//ios/chrome/common",
+      "//ios/chrome/common/physical_web",
+      "//ios/chrome/share_extension:packed_resources",
+      "//ios/chrome/today_extension:packed_resources",
+      "//ios/net:ios_net_unittests",
+      "//ios/public/provider/chrome/browser",
+      "//ios/public/provider/web",
+      "//ios/testing:ocmock_support_unittest",
+      "//ios/third_party/earl_grey",
+      "//ios/web:earl_grey_test_support",
+      "//ios/web:ios_web_inttests",
+      "//ios/web:ios_web_unittests",
+      "//ios/web/shell:ios_web_shell",
+      "//ios/web/shell/test:ios_web_shell_test_host",
+    ]
   }
 }