Add GN variables for controlling header checking.

tools/gn/functions_target.cc

 // 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.
 
 #include "tools/gn/functions.h"
 
 #include "tools/gn/config_values_generator.h"
 #include "tools/gn/err.h"
 #include "tools/gn/parse_tree.h"
 #include "tools/gn/scope.h"
@@ skipping 274 matching lines @@
   if (!FillTargetBlockScope(scope, function, component_mode.c_str(), block,
                             args, &block_scope, err))
     return Value();
 
   block->ExecuteBlockInScope(&block_scope, err);
   if (err->has_error())
     return Value();
 
   TargetGenerator::GenerateTarget(&block_scope, function, args,
                                   component_mode, err);
+  if (err->has_error())
+    return Value();
 
   block_scope.CheckForUnusedVars(err);
   return Value();
 }
 
 // copy ------------------------------------------------------------------------
 
 const char kCopy[] = "copy";
 const char kCopy_HelpShort[] =
     "copy: Declare a target that copies files.";
@@ skipping 235 matching lines @@
 Value RunTest(Scope* scope,
               const FunctionCallNode* function,
               const std::vector<Value>& args,
               BlockNode* block,
               Err* err) {
   return ExecuteGenericTarget(functions::kExecutable, scope, function, args,
                               block, err);
 }
 
 }  // namespace functions