gn: Support build directories outside the source tree.

tools/gn/path_output.h

Index: tools/gn/path_output.h
diff --git a/tools/gn/path_output.h b/tools/gn/path_output.h
index 12fc5ac5ea98dad5fdafbc7d91e190e6155f53a6..869ac46f4afbae09904916dfba3e6c3ec775f7ab 100644
--- a/tools/gn/path_output.h
+++ b/tools/gn/path_output.h
@@ -33,7 +33,8 @@ class PathOutput {
     DIR_NO_LAST_SLASH,
   };
 
-  PathOutput(const SourceDir& current_dir, EscapingMode escaping);
+  PathOutput(const SourceDir& current_dir, const base::FilePath& source_root,
+             EscapingMode escaping);
   ~PathOutput();
 
   // Read-only since inverse_current_dir_ is computed depending on this.