GLES 3.0.4 p147:
 "If internalformat is an ETC2/EAC format, CompressedTexImage3D will generate an
  INVALID_OPERATION error if target is not TEXTURE_2D_ARRAY."

This matches the validation of glTexImage3D.

TEST=conformance2/textures/misc/tex-storage-compressed-formats.html

BUG=chromium:983142

Change-Id: Ifc4ea9503182c43de7e58c4b45b59bb03ae2f901
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1698648Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>