GN: Fix dependency output file handling.

tools/gn/substitution_pattern.h

Index: tools/gn/substitution_pattern.h
diff --git a/tools/gn/substitution_pattern.h b/tools/gn/substitution_pattern.h
index 7e6c662519ad4518b25961480f1d298418e036ae..a3d6f7e633400c524492f6c2fb682a303edd9ed9 100644
--- a/tools/gn/substitution_pattern.h
+++ b/tools/gn/substitution_pattern.h
@@ -41,6 +41,10 @@ class SubstitutionPattern {
   bool Parse(const Value& value, Err* err);
   bool Parse(const std::string& str, const ParseNode* origin, Err* err);
 
+  // Makes a pattern given a hardcoded string. Will assert if the string is
+  // not a valid pattern.
+  static SubstitutionPattern MakeForTest(const char* str);
+
   // Returns the pattern as a string with substitutions in them.
   std::string AsString() const;