GN: error out if attempting to use_sysroot with gcc

build/config/sysroot.gni

Index: build/config/sysroot.gni
diff --git a/build/config/sysroot.gni b/build/config/sysroot.gni
index 2ca64518f6e23722d22fab6e6d31d01d2a074995..29a2b6117a30cb77da72b2ff24773f8d925d4b13 100644
--- a/build/config/sysroot.gni
+++ b/build/config/sysroot.gni
@@ -50,6 +50,12 @@ if (current_toolchain == default_toolchain && target_sysroot != "") {
   }
 
   if (sysroot != "") {
+    # Our sysroot images only contains gcc 4.6 headers, but chromium requires
+    # gcc 4.9. Clang is able to detect and work with the 4.6 headers while
+    # gcc is not. This check can be removed if we ever update to a more modern
+    # sysroot.
+    assert(is_clang, "sysroot images require clang (try use_sysroot=false)")
+
     _script_arch = current_cpu
     if (_script_arch == "x86") {
       _script_arch = "i386"