WebGLCompatibility implement and add tests for section 6.4 The checks for VertexAttribPointer were already implemented in a previous patch, so we only add the checks for DrawElements. BUG=angleproject:1523 BUG=chromium:668223 Change-Id: I5da55f9a7e7479627099c7f77618353a63b75a8e Reviewed-on: https://chromium-review.googlesource.com/434958 Commit-Queue: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>