i18n: Fix a crash because of unaligned memory accesses.

The buffer used in placement new should have the same alignment
as the underlying type DecimalFormatProperties.
Fixes login crashes in Chrome OS.

BUG=chromium:865100
TEST=locally built chrome os image works.

Change-Id: Ic71cb4e7962f1b9170e0a4ef00223767c621bc67
Reviewed-on: https://chromium-review.googlesource.com/1145582


Reviewed-by: Jungshik Shin <jshin@chromium.org>