Introduce new API for detecting when the embedder is in foreground/background.

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 9971bbb1017b75c0b0ed798a35ef223d0bb20522..0cef6591643735479706ed9af8ecfa5fe994f44e 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -1029,6 +1029,8 @@ class Heap {
   bool HasHighFragmentation();
   bool HasHighFragmentation(intptr_t used, intptr_t committed);
 
+  void SetOptimizeForLatency() { optimize_for_memory_usage_ = false; }
+  void SetOptimizeForMemoryUsage() { optimize_for_memory_usage_ = true; }
   bool ShouldOptimizeForMemoryUsage() { return optimize_for_memory_usage_; }
 
   // ===========================================================================