Fix compilation with Xcode version of clang.

build/toolchain/toolchain.gni

Index: build/toolchain/toolchain.gni
diff --git a/build/toolchain/toolchain.gni b/build/toolchain/toolchain.gni
index fbaf48066840ae7f51436e87ff8e52b23091fc4f..0dde8d9f1d1e4b20657f8fedc6224dfee1a64343 100644
--- a/build/toolchain/toolchain.gni
+++ b/build/toolchain/toolchain.gni
@@ -49,7 +49,10 @@ declare_args() {
   }
 }
 
-assert(!use_xcode_clang || is_ios,
+# Check target_os here instead of is_ios as this file is loaded for secondary
+# toolchain (host toolchain in particular) but the argument is the same for
+# all toolchains.
+assert(!use_xcode_clang || target_os == "ios",
        "Using Xcode's clang is only supported in iOS builds")
 
 # Subdirectory within root_out_dir for shared library files.