Improve GN handling of directory templates.

tools/gn/filesystem_utils.h

Index: tools/gn/filesystem_utils.h
diff --git a/tools/gn/filesystem_utils.h b/tools/gn/filesystem_utils.h
index 71e70570284f94eaadd15a005b94329915b13e9a..3864eccebbd9fce4124c447db8efe165d3f61624 100644
--- a/tools/gn/filesystem_utils.h
+++ b/tools/gn/filesystem_utils.h
@@ -104,11 +104,16 @@ base::StringPiece FindLastDirComponent(const SourceDir& dir);
 //
 // The originating value will be blamed in the error.
 //
+// Setting allow_templates indicates that the result will be put into a
+// FileTemplate and it will allow the strings to be prefixed with expansions
+// that would include the output path.
+//
 // If the file isn't in the dir, returns false and sets the error. Otherwise
 // returns true and leaves the error untouched.
 bool EnsureStringIsInOutputDir(const SourceDir& dir,
                                const std::string& str,
                                const Value& originating,
+                               bool allow_templates,
                                Err* err);
 
 // ----------------------------------------------------------------------------