Reduce input dependencies in GN.

tools/gn/ninja_action_target_writer.h

Index: tools/gn/ninja_action_target_writer.h
diff --git a/tools/gn/ninja_action_target_writer.h b/tools/gn/ninja_action_target_writer.h
index b440ac224935a458548a9ee372a84cac74f234e9..ac35b5b66477ce7f59c016c17596285d3ecb0e1b 100644
--- a/tools/gn/ninja_action_target_writer.h
+++ b/tools/gn/ninja_action_target_writer.h
@@ -39,10 +39,10 @@ class NinjaActionTargetWriter : public NinjaTargetWriter {
   // 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