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

build/config/linux/pkg_config.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.
 
 import("//build/config/sysroot.gni")
 
 # Defines a config specifying the result of running pkg-config for the given
 # packages. Put the package names you want to query in the "packages" variable
 # inside the template invocation.
 #
 # You can also add defines via the "defines" variable. This can be useful to
 # add this to the config to pass defines that the library expects to get by
 # users of its headers.
 #
 # Example:
 #   pkg_config("mything") {
 #     packages = [ "mything1", "mything2" ]
 #     defines = [ "ENABLE_AWESOME" ]
 #   }
 #
 # You can also use "extra args" to filter out results (see pkg-config.py):
 #   extra_args = [ "-v, "foo" ]
 # To ignore libs and ldflags (only cflags/defines will be set, which is useful
 # when doing manual dynamic linking), set:
 #   ignore_libs = true
 
+declare_args() {
+  # A pkg-config wrapper to call instead of trying to find and call the right
+  # pkg-config directly. Wrappers like this are common in cross-compilation
+  # environments.
+  # Leaving it blank defaults to searching PATH for 'pkg-config' and relying on
+  # the sysroot mechanism to find the right .pc files.
+  pkg_config = ""
+}
+
 template("pkg_config") {
   assert(defined(invoker.packages),
         "Variable |packages| must be defined to be a list in pkg_config.")
   config(target_name) {
     if (sysroot != "") {
       # Pass the sysroot if we're using one (it requires the CPU arch also).
       args = ["-s", sysroot, "-a", cpu_arch] + invoker.packages
+    } else if (pkg_config != "") {
+      args = ["-p", pkg_config] + invoker.packages
     } else {
       args = invoker.packages
     }
 
     if (defined(invoker.extra_args)) {
       args += invoker.extra_args
     }
 
     pkgresult = exec_script("//build/config/linux/pkg-config.py",
                             args, "value")
     include_dirs = pkgresult[0]
     cflags = pkgresult[1]
 
     if (!defined(invoker.ignore_libs) || !invoker.ignore_libs) {
       libs = pkgresult[2]
       lib_dirs = pkgresult[3]
       ldflags = pkgresult[4]
     }
 
     if (defined(invoker.defines)) {
       defines = invoker.defines
     }
     if (defined(invoker.visibility)) {
       visibility = invoker.visibility
     }
   }
 }