Fix ValidateFragmentShaderColorBufferMaskMatch for indexed masks Adjust WebGL-specific validation, add test. Bug: angleproject:4394 Change-Id: I4d51d6d7639a4ec3115025c858434491845440a2 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3673298 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>