Change default heap maximum size to be 256m.

Useful for command line benchmarks.

Change-Id: Ie525863cd8eff93c64ce76639b1108fbdad91633
diff --git a/runtime/gc/heap.h b/runtime/gc/heap.h
index 07db169..86dab21 100644
--- a/runtime/gc/heap.h
+++ b/runtime/gc/heap.h
@@ -120,7 +120,7 @@
 
   static constexpr size_t kDefaultStartingSize = kPageSize;
   static constexpr size_t kDefaultInitialSize = 2 * MB;
-  static constexpr size_t kDefaultMaximumSize = 32 * MB;
+  static constexpr size_t kDefaultMaximumSize = 256 * MB;
   static constexpr size_t kDefaultMaxFree = 2 * MB;
   static constexpr size_t kDefaultMinFree = kDefaultMaxFree / 4;
   static constexpr size_t kDefaultLongPauseLogThreshold = MsToNs(5);