Keep zone cached in SnapshotReader to allow removing ZoneHandle(Isolate*) interface.

runtime/vm/benchmark_test.cc

Index: runtime/vm/benchmark_test.cc
===================================================================
--- runtime/vm/benchmark_test.cc	(revision 44579)
+++ runtime/vm/benchmark_test.cc	(working copy)
@@ -534,7 +534,8 @@
     intptr_t buffer_len = writer.BytesWritten();
 
     // Read object back from the snapshot.
-    SnapshotReader reader(buffer, buffer_len, Snapshot::kMessage, isolate);
+    SnapshotReader reader(buffer, buffer_len, Snapshot::kMessage, isolate,
+                          zone.GetZone());
     reader.ReadObject();
   }
   timer.Stop();
@@ -557,7 +558,8 @@
     intptr_t buffer_len = writer.BytesWritten();
 
     // Read object back from the snapshot.
-    SnapshotReader reader(buffer, buffer_len, Snapshot::kMessage, isolate);
+    SnapshotReader reader(buffer, buffer_len, Snapshot::kMessage, isolate,
+                          zone.GetZone());
     reader.ReadObject();
   }
   timer.Stop();
@@ -582,7 +584,8 @@
     intptr_t buffer_len = writer.BytesWritten();
 
     // Read object back from the snapshot.
-    SnapshotReader reader(buffer, buffer_len, Snapshot::kMessage, isolate);
+    SnapshotReader reader(buffer, buffer_len, Snapshot::kMessage, isolate,
+                          zone.GetZone());
     reader.ReadObject();
     free(buffer);
   }