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/1343139Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>