Implement the WebGL restriction on reading from missing attachments
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 <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>
Commit-Queue: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>

ecoal95/angle,ecoal95/angle,ppy/angle,ppy/angle,MSOpenTech/angle,ppy/angle,ppy/angle,ecoal95/angle,ecoal95/angle,MSOpenTech/angle,ecoal95/angle,MSOpenTech/angle,MSOpenTech/angle