An incorrect call to SetWindowLongPtr was crashing the 64-bit test.

BUG=angle:705

Change-Id: I3815bb0a1308f5e66e74faead37c0ae09f975a10
Reviewed-on: https://chromium-review.googlesource.com/214883Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>