GN: Add ability to specify a depfile for custom targets.

tools/gn/ninja_script_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_SCRIPT_TARGET_WRITER_H_
 #define TOOLS_GN_NINJA_SCRIPT_TARGET_WRITER_H_
 
 #include <vector>
 
 #include "base/compiler_specific.h"
@@ skipping 10 matching lines @@
                           const Toolchain* toolchain,
                           std::ostream& out);
   virtual ~NinjaScriptTargetWriter();
 
   virtual void Run() OVERRIDE;
 
  private:
   FRIEND_TEST_ALL_PREFIXES(NinjaScriptTargetWriter,
                            WriteOutputFilesForBuildLine);
   FRIEND_TEST_ALL_PREFIXES(NinjaScriptTargetWriter,
+                           WriteOutputFilesForBuildLineWithDepfile);
+  FRIEND_TEST_ALL_PREFIXES(NinjaScriptTargetWriter,
                            WriteArgsSubstitutions);
 
   bool has_sources() const { return !target_->sources().empty(); }
 
   // Writes the Ninja rule for invoking the script.
   //
   // Returns the name of the custom rule generated. This will be based on the
   // target name, and will include the string "$unique_name" if there are
   // multiple inputs.
   std::string WriteRuleDefinition(const FileTemplate& args_template);
@@ skipping 17 matching lines @@
   // of the script into one thing that other targets can depend on.
   void WriteStamp(const std::vector<OutputFile>& output_files);
 
   // Writes the output files generated by the output template for the given
   // source file. This will start with a space and will not include a newline.
   // Appends the output files to the given vector.
   void WriteOutputFilesForBuildLine(const FileTemplate& output_template,
                                     const SourceFile& source,
                                     std::vector<OutputFile>* output_files);
 
+  void WriteDepfile(const SourceFile& source);
+
+  // Returns the FileTemplate for the depfile variable.
+  FileTemplate GetDepfileTemplate() const;
+
   // Path output writer that doesn't do any escaping or quoting. It does,
   // however, convert slashes.  Used for
   // computing intermediate strings.
   PathOutput path_output_no_escaping_;
 
   DISALLOW_COPY_AND_ASSIGN(NinjaScriptTargetWriter);
 };
 
 #endif  // TOOLS_GN_NINJA_SCRIPT_TARGET_WRITER_H_