Add config to enable ARC on iOS/OS X to //build/config.

Add a new config //build/config/compiler:enable_arc that adds the
required flag to enable ARC (automatic reference counting).

BUG=594519

Review URL: https://codereview.chromium.org/1806513002

Cr-Commit-Position: refs/heads/master@{#381686}

build/config/compiler/BUILD.gn

@@ -1335,3 +1335,13 @@ config("default_symbols") {
     assert(false)
   }
 }
+
+if (is_ios || is_mac) {
+  # On Mac and iOS, this enables support for ARC (automatic ref-counting).
+  # See http://clang.llvm.org/docs/AutomaticReferenceCounting.html.
+  config("enable_arc") {
+    common_flags = [ "-fobjc-arc" ]
+    cflags_objc = common_flags
+    cflags_objcc = common_flags
+  }
+}

ios/third_party/gcdwebserver/BUILD.gn

@@ -2,7 +2,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-config("public_config") {
+config("config") {
+  visibility = [ ":gcdwebserver" ]
   libs = [
     "CFNetwork.framework",
     "MobileCoreServices.framework",
@@ -16,14 +17,13 @@ config("public_config") {
   ]
 }
 
-config("internal_config") {
+config("private_config") {
   visibility = [ ":gcdwebserver" ]
 
   # TODO(crbug.com/569158): Suppresses warnings that are treated as errors
   # when minimum iOS version support is increased to iOS 9 and up.
   # This should be removed once all deprecation violations have been fixed.
   cflags = [ "-Wno-deprecated-declarations" ]
-  cflags_objc = [ "-fobjc-arc" ]
 }
 
 source_set("gcdwebserver") {
@@ -60,6 +60,9 @@ source_set("gcdwebserver") {
     "src/GCDWebServer/Responses/GCDWebServerStreamedResponse.m",
   ]
 
-  public_configs = [ ":public_config" ]
-  configs += [ ":internal_config" ]
+  public_configs = [ ":config" ]
+  configs += [
+    ":private_config",
+    "//build/config/compiler:enable_arc",
+  ]
 }

ios/third_party/ochamcrest/BUILD.gn

@@ -175,25 +175,20 @@ copy("ochamcrest_copy_files") {
   ]
 }
 
-config("ochamcrest_config") {
+config("config") {
   visibility = [ ":ochamcrest" ]
   libs = [ "XCTest.framework" ]
   include_dirs = [ "$root_gen_dir/ios/third_party/ochamcrest" ]
 }
 
-config("ochamcrest_private_config") {
-  visibility = [ ":ochamcrest" ]
-  cflags_objc = [ "-fobjc-arc" ]
-}
-
 source_set("ochamcrest") {
   testonly = true
   configs += [
-    ":ochamcrest_config",
-    ":ochamcrest_private_config",
+    ":config",
+    "//build/config/compiler:enable_arc",
     "//build/config/compiler:no_chromium_code",
   ]
-  public_configs = [ ":ochamcrest_config" ]
+  public_configs = [ ":config" ]
   public_deps = [
     ":ochamcrest_copy_files",
   ]