Fix wrong state of IsolateInBackgroundNotification and IsolateInForegroundNotification.

BUG=chromium:636368,chromium:635965,chromium:634900

Review-Url: https://codereview.chromium.org/2245483004
Cr-Commit-Position: refs/heads/master@{#38598}

src/isolate.cc

@@ -3174,12 +3174,12 @@ void Isolate::SetRAILMode(RAILMode rail_mode) {
 }
 
 void Isolate::IsolateInBackgroundNotification() {
-  is_isolate_in_background_ = false;
+  is_isolate_in_background_ = true;
   heap()->ActivateMemoryReducerIfNeeded();
 }
 
 void Isolate::IsolateInForegroundNotification() {
-  is_isolate_in_background_ = true;
+  is_isolate_in_background_ = false;
 }
 
 bool StackLimitCheck::JsHasOverflowed(uintptr_t gap) const {