Remove performanceMonitor()->unsubscribeAll() from Performance's destructor

Performance object is on Oilpan's heap, so its destructor is not allowed to touch
other on-heap objects.

Also we don't need to call performanceMonitor()->unsubscribeAll() because it is
just unregistering a back pointer Member to the Performance object from the map
in PerformanceMonitor.

BUG=

Review-Url: https://codereview.chromium.org/2584733003
Cr-Commit-Position: refs/heads/master@{#439081}

third_party/WebKit/Source/core/timing/Performance.cpp

@@ -104,8 +104,9 @@ Performance::Performance(LocalFrame* frame)
       ContextLifecycleObserver(frame ? frame->document() : nullptr) {}
 
 Performance::~Performance() {
-  if (frame())
-    frame()->performanceMonitor()->unsubscribeAll(this);
+  // contextDestryoed() should have been called and the performance
+  // monitors should have been unsubscribed.
+  DCHECK(!frame());
 }
 
 void Performance::contextDestroyed() {