Allow a custom toolchain to be injected into BUILDCONFIG.gn

build/config/BUILDCONFIG.gn

Index: build/config/BUILDCONFIG.gn
diff --git a/build/config/BUILDCONFIG.gn b/build/config/BUILDCONFIG.gn
index 234332b8547e95b0185e9bc4bcaaf1c1bfedcf63..02618768053ede93ffb208e98eb1748d6102382e 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -125,12 +125,9 @@ declare_args() {
   is_clang = current_os == "mac" || current_os == "ios" ||
              current_os == "linux" || current_os == "chromeos"
 
-  if (current_os == "chromeos") {
-    # Allows the target toolchain to be injected as arguments. This is needed
-    # to support the CrOS build system which supports per-build-configuration
-    # toolchains.
-    cros_use_custom_toolchain = false
-  }
+  # Allows the path to a custom target toolchain to be injected as a single
+  # argument, and set as the default toolchain.
+  custom_toolchain = ""
 
   # DON'T ADD MORE FLAGS HERE. Read the comment above.
 }
@@ -489,6 +486,7 @@ set_defaults("test") {
 # doing cross-compiles. When not cross-compiling, this will be the same as the
 # default toolchain.
 
+_default_toolchain = ""
 if (is_win) {
   # On windows we use the same toolchain for host and target by default.
   if (is_clang) {
@@ -498,11 +496,11 @@ if (is_win) {
   }
 
   if (current_os == "win") {
-    set_default_toolchain("$host_toolchain")
+    _default_toolchain = host_toolchain
   } else if (current_cpu == "x64") {  # WinRT x64
-    set_default_toolchain("//build/toolchain/win:winrt_x64")
+    _default_toolchain = "//build/toolchain/win:winrt_x64"
   } else if (current_cpu == "x86") {  # WinRT x86
-    set_default_toolchain("//build/toolchain/win:winrt_x86")
+    _default_toolchain = "//build/toolchain/win:winrt_x86"
   }
 } else if (is_android) {
   if (host_os == "linux") {
@@ -518,27 +516,24 @@ if (is_win) {
     assert(false, "Unknown host for android cross compile")
   }
   if (is_clang) {
-    set_default_toolchain("//build/toolchain/android:clang_$current_cpu")
+    _default_toolchain = "//build/toolchain/android:clang_$current_cpu"
   } else {
-    set_default_toolchain("//build/toolchain/android:$current_cpu")
+    _default_toolchain = "//build/toolchain/android:$current_cpu"
   }
 } else if (is_linux) {
   if (is_clang) {
     host_toolchain = "//build/toolchain/linux:clang_$host_cpu"
-    set_default_toolchain("//build/toolchain/linux:clang_$current_cpu")
+    _default_toolchain = "//build/toolchain/linux:clang_$current_cpu"
   } else {
     host_toolchain = "//build/toolchain/linux:$host_cpu"
-    set_default_toolchain("//build/toolchain/linux:$current_cpu")
-  }
-  if (is_chromeos && cros_use_custom_toolchain) {
-    set_default_toolchain("//build/toolchain/cros:target")
+    _default_toolchain = "//build/toolchain/linux:$current_cpu"
   }
 } else if (is_mac) {
   host_toolchain = "//build/toolchain/mac:clang_x64"
-  set_default_toolchain(host_toolchain)
+  _default_toolchain = host_toolchain
 } else if (is_ios) {
   host_toolchain = "//build/toolchain/mac:clang_x64"
-  set_default_toolchain("//build/toolchain/mac:ios_clang_arm")
+  _default_toolchain = "//build/toolchain/mac:ios_clang_arm"
 } else if (is_nacl) {
   # TODO(GYP): This will need to change when we get NaCl working
   # on multiple platforms, but this whole block of code (how we define
@@ -547,6 +542,14 @@ if (is_win) {
   host_toolchain = "//build/toolchain/linux:clang_x64"
 }
 
+# If a custom toolchain has been set in the args, set it as default. Otherwise,
+# set the default toolchain for the platform (if any).
+if (custom_toolchain != "") {
+  set_default_toolchain(custom_toolchain)
+} else if (_default_toolchain != "") {
+  set_default_toolchain(_default_toolchain)
+}
+
 # ==============================================================================
 # COMPONENT SETUP
 # ==============================================================================