Split nested namespaces declaration in two lines in accordance with C++ Style Guide.

src/mark-compact.h

Index: src/mark-compact.h
diff --git a/src/mark-compact.h b/src/mark-compact.h
index bfa2c3ce5184c97ad2b75b4bb3ed4c55ff586091..d7ad630136ba92f0b295ef8d416e527a0849aab7 100644
--- a/src/mark-compact.h
+++ b/src/mark-compact.h
@@ -28,7 +28,8 @@
 #ifndef V8_MARK_COMPACT_H_
 #define V8_MARK_COMPACT_H_
 
-namespace v8 { namespace internal {
+namespace v8 {
+namespace internal {
 
 // Callback function, returns whether an object is alive. The heap size
 // of the object is returned in size. It optionally updates the offset