Adds a file to track known failures in the WebGL CTS

Because ANGLE does not have a framework for testing directly against
the WebGL CTS, we're unable to suppress failing tests at runtime. Instead,
we'll document them in the repository for the reference of contributors.

Change-Id: Ieea68a9aaa0b7bfebda2900265e3c4a857a0ce64
Reviewed-on: https://chromium-review.googlesource.com/230582
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Shannon Woods <shannonwoods@chromium.org>