Added tracking of memory usage within ApiNativeScopes. Usage to be displayed within Observatory.

runtime/vm/dart_api_state.h

Index: runtime/vm/dart_api_state.h
diff --git a/runtime/vm/dart_api_state.h b/runtime/vm/dart_api_state.h
index 3ca4f728b0da255ed68ab03ba171bb7cbb50a1b6..eb7ad36621e6f174f6a4f9dae6b9d963686b93bb 100644
--- a/runtime/vm/dart_api_state.h
+++ b/runtime/vm/dart_api_state.h
@@ -686,6 +686,16 @@ class ApiNativeScope {
         OSThread::GetThreadLocal(Api::api_native_key_));
   }
 
+  static intptr_t current_memory_usage() { return current_memory_usage_; }
+
+  static void IncrementNativeScopeMemoryUsage(intptr_t size) {
+    AtomicOperations::IncrementBy(&current_memory_usage_, size);
+  }
+
+  static void DecrementNativeScopeMemoryUsage(intptr_t size) {
+    AtomicOperations::DecrementBy(&current_memory_usage_, size);
+  }
+
   Zone* zone() {
     Zone* result = zone_.GetZone();
     ASSERT(result->handles()->CountScopedHandles() == 0);
@@ -694,6 +704,9 @@ class ApiNativeScope {
   }
 
  private:
+  // The current total memory usage within ApiNativeScopes.
+  static intptr_t current_memory_usage_;
+
   ApiZone zone_;
 };