Remove observer when element array buffer is detached Fix an issue where the observer is not removed when the element array buffer is detached from the vertex array. This fixes use-after-free vulnerability. Bug: angleproject:7237 Change-Id: I7cf30322740edffccdba89d40c0d02cd351dad78 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3603007 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>