gn: Support build directories outside the source tree.

tools/gn/visibility.h

Index: tools/gn/visibility.h
diff --git a/tools/gn/visibility.h b/tools/gn/visibility.h
index 2e1b680fc48733551681a8b54fdff7ac22bc1ff8..8ce9ed10cdc724c75a9f26c5727cd72c4efe04e3 100644
--- a/tools/gn/visibility.h
+++ b/tools/gn/visibility.h
@@ -26,7 +26,8 @@ class Visibility {
 
   // Set the visibility to the thing specified by the given value. On failure,
   // returns false and sets the error.
-  bool Set(const SourceDir& current_dir, const Value& value, Err* err);
+  bool Set(const base::FilePath& source_root,
+           const SourceDir& current_dir, const Value& value, Err* err);
 
   // Sets the visibility to be public.
   void SetPublic();