GN: Add an assert that sysroot exists

build/config/sysroot.gni

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This header file defines the "sysroot" variable which is the absolute path
 # of the sysroot. If no sysroot applies, the variable will be an empty string.
 
 import("//build/config/chrome_build.gni")
 
 declare_args() {
@@ skipping 30 matching lines @@
   } else if (current_cpu == "x86") {
     sysroot = rebase_path("//build/linux/debian_wheezy_i386-sysroot")
   } else if (current_cpu == "mipsel") {
     sysroot = rebase_path("//build/linux/debian_wheezy_mips-sysroot")
   } else if (current_cpu == "arm") {
     sysroot = rebase_path("//build/linux/debian_wheezy_arm-sysroot")
   } else {
     # Any other builds don't use a sysroot.
     sysroot = ""
   }
+
+  if (sysroot != "") {
+    _script_arch = current_cpu
+    if (_script_arch == "x86") {
+      _script_arch = "i386"
+    } else if (_script_arch == "x64") {
+      _script_arch = "amd64"
+    }
+    assert(
+        exec_script("//build/dir_exists.py", [ sysroot ], "string") == "True",
+        "Missing sysroot ($sysroot). To fix, run: build/linux/sysroot_scripts/install-sysroot.py --arch=$_script_arch")
+  }
 } else if (is_mac) {
   import("//build/config/mac/mac_sdk.gni")
   sysroot = mac_sdk_path
 } else if (is_ios) {
   import("//build/config/ios/ios_sdk.gni")
   sysroot = ios_sdk_path
 } else {
   sysroot = ""
 }