-[NSWindow convertRectToScreen] was introduced in OSX 10.7, use
convertBaseToScreen instead for OSX 10.6

BUG=angleproject:891

Change-Id: I255a71d98bc9bf22735a7cb8b9ce1ea24061742f
Reviewed-on: https://chromium-review.googlesource.com/299722Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>