gn: Support build directories outside the source tree.

tools/gn/label_pattern.cc

Index: tools/gn/label_pattern.cc
diff --git a/tools/gn/label_pattern.cc b/tools/gn/label_pattern.cc
index 752898f1acace2f03bdddc25315da680728b6d00..ff6b34af0751b57fe84e051448e189a0ad06ed04 100644
--- a/tools/gn/label_pattern.cc
+++ b/tools/gn/label_pattern.cc
@@ -166,7 +166,7 @@ LabelPattern LabelPattern::GetPattern(const SourceDir& current_dir,
     }
 
     // Resolve the non-wildcard stuff.
-    dir = current_dir.ResolveRelativeDir(path);
+    dir = current_dir.ResolveRelativeDir(path, base::FilePath());
     if (dir.is_null()) {
       *err = Err(value, "Label pattern didn't resolve to a dir.",
           "The directory name \"" + path.as_string() + "\" didn't\n"