Also _WIN64 implies _WIN32.

Bug: angleproject:2858
Change-Id: I63e2ffd2e9e304171ea6adb99836733981cc1813
Reviewed-on: https://chromium-review.googlesource.com/1248441
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>