Implement the WebGL restriction on reading from missing attachments BUG=angleproject:1523 BUG=chromium:668223 Change-Id: I2dffa3c92dd32e384d3b3109084420650ca7f795 Reviewed-on: https://chromium-review.googlesource.com/421113 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Corentin Wallez <cwallez@chromium.org>