This includes an implementation of a cross platform Sleep function

BUG=angleproject:892

Change-Id: I1087a00ab204b37bafc5e95a9766962b194d97ef
Reviewed-on: https://chromium-review.googlesource.com/273133Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>