Add support for multi-architecture application bundle on iOS.

build/config/BUILDCONFIG.gn

Index: build/config/BUILDCONFIG.gn
diff --git a/build/config/BUILDCONFIG.gn b/build/config/BUILDCONFIG.gn
index 4f3a223c32d23ab7b79007add5081deac14bdf7e..a6018a95a10aeea0b0e7e9532ad4d5e849358818 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -2,6 +2,15 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+if (target_os == "ios") {
+  declare_args() {
+    # If defined, then the build is a multi-architecture build (aka fat build)
+    # and target_cpu must not be defined. Any value supported by target_cpu can
+    # be used in this list.
+    multi_arch_target_cpus = []
+  }
+}
+
 # =============================================================================
 # PLATFORM SELECTION
 # =============================================================================
@@ -45,6 +54,18 @@ if (target_cpu == "") {
     # This allows us to not have to specify both target_os and target_cpu
     # on the command line.
     target_cpu = "arm"
+  } else if (target_os == "ios") {
+    # If we're building for iOS, we respect the multi_arch_target_cpus if
+    # it is defined. In that case, we pick the first architecture listed in
+    # multi_arch_target_cpus and use it as target_cpu, the others will be
+    # build as secondary toolchain by the ios_app_bundle target.
+    if (multi_arch_target_cpus != []) {
+      assert(target_cpu == "",
+             "cannot define both multi_arch_target_cpus and target_cpu")
+      target_cpu = multi_arch_target_cpus[0]
+    } else {
+      target_cpu = host_cpu
+    }
   } else {
     target_cpu = host_cpu
   }
@@ -214,6 +235,20 @@ if (target_os == "android") {
   }
 } else if (target_os == "ios") {
   _default_toolchain = "//build/toolchain/mac:ios_clang_$target_cpu"
+  assert(custom_toolchain == "" || multi_arch_target_cpus == [],
+         "cannot define both multi_arch_target_cpus and custom_toolchain")
+
+  # Initialize multi_arch_toolchains from multi_arch_target_cpus. The
+  # list will not contain the default_toolchain and may be empty while
+  # multi_arch_target_cpus is not. This allows the build to default to
+  # regular build if multi_arch_target_cpus contains a single cpu.
+  multi_arch_toolchains = []
+  foreach(_multi_arch_target_cpu, multi_arch_target_cpus) {
+    if (_multi_arch_target_cpu != target_cpu) {
+      multi_arch_toolchains +=
+          [ "//build/toolchain/mac:ios_clang_$_multi_arch_target_cpu" ]
+    }
+  }
 } else if (target_os == "linux") {
   if (is_clang) {
     _default_toolchain = "//build/toolchain/linux:clang_$target_cpu"