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

runtime/vm/thread.h

Index: runtime/vm/thread.h
diff --git a/runtime/vm/thread.h b/runtime/vm/thread.h
index 2f1b4dff014ca3495615400ff215f69fc1fe7084..c5a0c8f3f148b2c56380f9833c48ffb5c77fbb8d 100644
--- a/runtime/vm/thread.h
+++ b/runtime/vm/thread.h
@@ -577,9 +577,9 @@ LEAF_RUNTIME_ENTRY_LIST(DECLARE_MEMBERS)
 #undef REUSABLE_HANDLE_SCOPE_VARIABLE
 #endif  // defined(DEBUG)
 
-  class AtSafepointField : public BitField<bool, 0, 1> {};
-  class SafepointRequestedField : public BitField<bool, 1, 1> {};
-  class BlockedForSafepointField : public BitField<bool, 2, 1> {};
+  class AtSafepointField : public BitField<uint32_t, bool, 0, 1> {};
+  class SafepointRequestedField : public BitField<uint32_t, bool, 1, 1> {};
+  class BlockedForSafepointField : public BitField<uint32_t, bool, 2, 1> {};
   uint32_t safepoint_state_;
   uint32_t execution_state_;