Reduce input dependencies in GN.

tools/gn/ninja_action_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_ACTION_TARGET_WRITER_H_
 #define TOOLS_GN_NINJA_ACTION_TARGET_WRITER_H_
 
 #include <vector>
 
 #include "base/compiler_specific.h"
@@ skipping 21 matching lines @@
   // 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();
 
   // Writes the rules for compiling each source, writing all output files
   // to the given vector.
   //
-  // implicit_deps is a precomputed string of all ninja files that are common
-  // to each build step, it starts with a "|" if it's nonempty.
+  // input_dep is a file expressing the depencies common to all build steps.
+  // It will be a stamp file if there is more than one.
   void WriteSourceRules(const std::string& custom_rule_name,
-                        const std::string& implicit_deps,
+                        const OutputFile& input_dep,
                         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 SourceFile& source,
                                     std::vector<OutputFile>* output_files);
 
   void WriteDepfile(const SourceFile& source);
 
   // 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(NinjaActionTargetWriter);
 };
 
 #endif  // TOOLS_GN_NINJA_ACTION_TARGET_WRITER_H_