Otherwise apitrace doesn't know the dimension of the framebuffer and
cannot display it.

BUG=angleproject:892

Change-Id: Ib8ad846ca80e4783ab77c96430bb5220df1a39ab
Reviewed-on: https://chromium-review.googlesource.com/286881Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>