GN: Throw an error for duplicate object files.

tools/gn/ninja_binary_target_writer.h

 // 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.
 
 #ifndef TOOLS_GN_NINJA_BINARY_TARGET_WRITER_H_
 #define TOOLS_GN_NINJA_BINARY_TARGET_WRITER_H_
 
 #include "base/macros.h"
 #include "tools/gn/config_values.h"
 #include "tools/gn/ninja_target_writer.h"
@@ skipping 98 matching lines @@
   //
   // The order-only dependencies are the non-linkable deps passed in as an
   // argument, plus the data file depdencies in the target.
   void WriteOrderOnlyDependencies(
       const UniqueVector<const Target*>& non_linkable_deps);
 
   // Returns the computed name of the Windows .pch file for the given
   // tool type. The tool must support precompiled headers.
   OutputFile GetWindowsPCHFile(Toolchain::ToolType tool_type) const;
 
+  // Checks for duplicates in the given list of output files. If any duplicates
+  // are found, throws an error and return false.
+  bool CheckForDuplicateObjectFiles(const std::vector<OutputFile>& files) const;
+
   const Tool* tool_;
 
   // Cached version of the prefix used for rule types for this toolchain.
   std::string rule_prefix_;
 
   DISALLOW_COPY_AND_ASSIGN(NinjaBinaryTargetWriter);
 };
 
 #endif  // TOOLS_GN_NINJA_BINARY_TARGET_WRITER_H_