GN: Fix dependency output file handling.

tools/gn/substitution_pattern.cc

Index: tools/gn/substitution_pattern.cc
diff --git a/tools/gn/substitution_pattern.cc b/tools/gn/substitution_pattern.cc
index 6a4d0c59158130f174d4d468290f4f0384ff9219..cb1bd24ef082d1103364e60f291b6419eb521979 100644
--- a/tools/gn/substitution_pattern.cc
+++ b/tools/gn/substitution_pattern.cc
@@ -91,6 +91,14 @@ bool SubstitutionPattern::Parse(const std::string& str,
   return true;
 }
 
+// static
+SubstitutionPattern SubstitutionPattern::MakeForTest(const char* str) {
+  Err err;
+  SubstitutionPattern pattern;
+  CHECK(pattern.Parse(str, NULL, &err)) << err.message();
+  return pattern;
+}
+
 std::string SubstitutionPattern::AsString() const {
   std::string result;
   for (size_t i = 0; i < ranges_.size(); i++) {