Fix some more shorten-64-to-32 warnings:

runtime/vm/class_table.h

Index: runtime/vm/class_table.h
diff --git a/runtime/vm/class_table.h b/runtime/vm/class_table.h
index aaabdf461792d2380d9b82d22853c9ab00625d8f..aa8c3660148c32118331f504d68118ec292da681 100644
--- a/runtime/vm/class_table.h
+++ b/runtime/vm/class_table.h
@@ -132,7 +132,8 @@ class ClassHeapStats {
     kTraceAllocationBit = 0,
   };
 
-  class TraceAllocationBit : public BitField<bool, kTraceAllocationBit, 1> {};
+  class TraceAllocationBit :
+      public BitField<intptr_t, bool, kTraceAllocationBit, 1> {};
 
   // Recent old at start of last new GC (used to compute promoted_*).
   intptr_t old_pre_new_gc_count_;