GN: Fix compile errors with os==chromeos mojo/public build

build/config/BUILDCONFIG.gn

Index: build/config/BUILDCONFIG.gn
diff --git a/build/config/BUILDCONFIG.gn b/build/config/BUILDCONFIG.gn
index bf2076cf1b6ef7e88bd83f5ca9b0395f301ad3f1..7994dd77f78ac7470c173718d9219cf9417e0ece 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -64,6 +64,13 @@ declare_args() {
 
   # Compile for Thread Sanitizer to find threading bugs.
   is_tsan = false
+
+  if (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
+  }
 }
 
 # =============================================================================
@@ -496,6 +503,9 @@ if (is_win) {
     host_toolchain = "//build/toolchain/linux:$build_cpu_arch"
     set_default_toolchain("//build/toolchain/linux:$cpu_arch")
   }
+  if (is_chromeos && cros_use_custom_toolchain) {
+    set_default_toolchain("//build/toolchain/cros:target")
+  }
 } else if (is_mac) {
   host_toolchain = "//build/toolchain/mac:clang"
   set_default_toolchain(host_toolchain)