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..e3a324f56568ba91100fb3c1087be8b72273b7c4 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -132,9 +132,26 @@ declare_args() {
     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.
+  # Note: On ChromeOS, do not set this argument. Instead, set
+  # cros_use_custom_toolchain, and set each of the args required by
+  # //build/toolchain/cros:target.
+  custom_toolchain = ""
+
   # DON'T ADD MORE FLAGS HERE. Read the comment above.
 }
 
+if (current_os == "chromeos") {
+  assert(
+      custom_toolchain == "",
+      "|custom_toolchain| should not be set in args on ChromeOS. Set cros_use_custom_toolchain instead.")
+
+  if (cros_use_custom_toolchain) {
+    custom_toolchain = "//build/toolchain/cros:target"
+  }
+}
+
 # =============================================================================
 # OS DEFINITIONS
 # =============================================================================
@@ -530,9 +547,6 @@ if (is_win) {
     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")
-  }
 } else if (is_mac) {
   host_toolchain = "//build/toolchain/mac:clang_x64"
   set_default_toolchain(host_toolchain)
@@ -547,6 +561,11 @@ if (is_win) {
   host_toolchain = "//build/toolchain/linux:clang_x64"
 }
 
+# Set the custom default toolchain.
+if (custom_toolchain != "") {
+  set_default_toolchain(custom_toolchain)
+}
+
 # ==============================================================================
 # COMPONENT SETUP
 # ==============================================================================