Moved zones and zone related stuff in its own directory.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 8ae8fbb8117a9abba36fa2ab02f41f499a1814af..9fa59efbe969f878a569a0fc3a65c29e6b6c84c2 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -8240,8 +8240,7 @@ void Isolate::IsolateInBackgroundNotification() {
 
 void Isolate::MemoryPressureNotification(MemoryPressureLevel level) {
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);
-  return isolate->heap()->MemoryPressureNotification(level,
-                                                     Locker::IsLocked(this));
+  isolate->heap()->MemoryPressureNotification(level, Locker::IsLocked(this));
 }
 
 void Isolate::SetRAILMode(RAILMode rail_mode) {