Fix GN action target writing with no sources list.

tools/gn/substitution_writer.cc

Index: tools/gn/substitution_writer.cc
diff --git a/tools/gn/substitution_writer.cc b/tools/gn/substitution_writer.cc
index 0738b6f7e0a070fa23092ea6049fb65e26e85e36..2b6245d5051ded9643b22fab00610fcf622be955 100644
--- a/tools/gn/substitution_writer.cc
+++ b/tools/gn/substitution_writer.cc
@@ -125,6 +125,40 @@ SubstitutionWriter::~SubstitutionWriter() {
 }
 
 // static
+void SubstitutionWriter::GetListAsSourceFiles(
+    const Settings* settings,
+    const SubstitutionList& list,
+    std::vector<SourceFile>* output) {
+  for (size_t i = 0; i < list.list().size(); i++) {
+    const SubstitutionPattern& pattern = list.list()[i];
+    CHECK(pattern.ranges().size() == 1 &&
+          pattern.ranges()[0].type == SUBSTITUTION_LITERAL)
+        << "The substitution patterm \""
+        << pattern.AsString()
+        << "\" was expected to be a literal with no {{substitutions}}.";
+    const std::string& literal = pattern.ranges()[0].literal;
+    CHECK(literal.size() >= 1 && literal[0] == '/')
+        << "The result of the pattern \""
+        << pattern.AsString()
+        << "\" was not an absolute path.";
+    output->push_back(SourceFile(literal));
+  }
+}
+
+void SubstitutionWriter::GetListAsOutputFiles(
+    const Settings* settings,
+    const SubstitutionList& list,
+    std::vector<OutputFile>* output) {
+  std::vector<SourceFile> output_as_sources;
+  GetListAsSourceFiles(settings, list, &output_as_sources);
+  for (size_t i = 0; i < output_as_sources.size(); i++) {
+    output->push_back(OutputFile(
+        RebaseSourceAbsolutePath(output_as_sources[i].value(),
+                                 settings->build_settings()->build_dir())));
+  }
+}
+
+// static
 SourceFile SubstitutionWriter::ApplyPatternToSource(
       const Settings* settings,
       const SubstitutionPattern& pattern,