gn: Support build directories outside the source tree.

tools/gn/label_pattern.h

Index: tools/gn/label_pattern.h
diff --git a/tools/gn/label_pattern.h b/tools/gn/label_pattern.h
index e805ffb62ef60dc82419f4baf6dbd9528ed6923b..bcc77d9b91d9cbe44fde56697499c8821bc738a1 100644
--- a/tools/gn/label_pattern.h
+++ b/tools/gn/label_pattern.h
@@ -35,7 +35,8 @@ class LabelPattern {
 
   // Converts the given input string to a pattern. This does special stuff
   // to treat the pattern as a label. Sets the error on failure.
-  static LabelPattern GetPattern(const SourceDir& current_dir,
+  static LabelPattern GetPattern(const base::FilePath& source_root,
+                                 const SourceDir& current_dir,
                                  const Value& value,
                                  Err* err);