WebGL: Fix DrawElements test and remove size check. Also optimizes the DrawElements validation to remove redundant tests. Also make some more validation WebGL-specific. Bug: angleproject:2979 Change-Id: I2d5c3094ed86ebadbc572e46b8ae105431f7ae68 Reviewed-on: https://chromium-review.googlesource.com/c/1343139 Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>