Add Heap::RunningOnValgrind and call it from the spaces.

Makes it easier to disable valgrind support.

Change-Id: I1bde792f1b76a2dd968fa03c6142e92fcc3670b0
diff --git a/runtime/gc/heap.h b/runtime/gc/heap.h
index eb53ba9..f650005 100644
--- a/runtime/gc/heap.h
+++ b/runtime/gc/heap.h
@@ -553,6 +553,9 @@
   void RemoveRememberedSet(space::Space* space);
 
   bool IsCompilingBoot() const;
+  bool RunningOnValgrind() const {
+    return running_on_valgrind_;
+  }
   bool HasImageSpace() const;
 
  private: