set old_gen_heap_size to half of available memory on iOS (#20472)

common/constants.h

diff --git a/common/constants.h b/common/constants.h
new file mode 100644
index 0000000..50b0f91
--- /dev/null
+++ b/common/constants.h
@@ -0,0 +1,7 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+namespace flutter {
+constexpr double kMegaByteSizeInBytes = (1 << 20);
+}  // namespace flutter