This make the multi window sample work well.

BUG=angleproject:892

Change-Id: If98c6f995875b63c391cee9b6fddc6d138d48f8b
Reviewed-on: https://chromium-review.googlesource.com/272946Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>