The ES3 spec is clear that these calls should no-op, see page 187,

"If a buffer is not present, then a Clear directed at that buffer has
no effect."

BUG=angleproject:1078
TEST=dEQP-GLES3.functional.fbo.msaa.2_samples.*

Change-Id: Ic521f55785d10065e2ffe2e98a68c9d7da0c2c71
Reviewed-on: https://chromium-review.googlesource.com/286776Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>