Work on GN GYP host build.

tools/gn/loader.cc

Index: tools/gn/loader.cc
diff --git a/tools/gn/loader.cc b/tools/gn/loader.cc
index e0a0517882ff96b031ef12fe5df558c83438dc27..1384b925542ad00c2d1b18b4a26d9c7e74cf491d 100644
--- a/tools/gn/loader.cc
+++ b/tools/gn/loader.cc
@@ -186,8 +186,8 @@ Label LoaderImpl::GetDefaultToolchain() const {
   return default_toolchain_label_;
 }
 
-const Settings* LoaderImpl::GetToolchainSettings(const Label& label) {
-  ToolchainRecordMap::iterator found_toolchain;
+const Settings* LoaderImpl::GetToolchainSettings(const Label& label) const {
+  ToolchainRecordMap::const_iterator found_toolchain;
   if (label.is_null()) {
     if (default_toolchain_label_.is_null())
       return NULL;