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

tools/gn/ninja_script_target_writer.h

Index: tools/gn/ninja_script_target_writer.h
diff --git a/tools/gn/ninja_script_target_writer.h b/tools/gn/ninja_script_target_writer.h
index e84d618ad58960cadca63e665dc421cf8fc0335b..b9f538b75fcdaea4eac1e897ab28b74a8841bf50 100644
--- a/tools/gn/ninja_script_target_writer.h
+++ b/tools/gn/ninja_script_target_writer.h
@@ -26,6 +26,8 @@ class NinjaScriptTargetWriter : public NinjaTargetWriter {
   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(); }
@@ -63,6 +65,11 @@ class NinjaScriptTargetWriter : public NinjaTargetWriter {
                                     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.