TextureD3D_Cube::isFaceLevelComplete skips checks for the base level
because it assumes that isCubeComplete has already been called to
verify the base level.  This assumption is not correct for some cases
like glCopyTextureCHROMIUM which attempts to create a render target
from cube maps that may not be complete.

This caused us to create render targets from incomplete cube maps and
then fail to upload data to the cube faces because the sizes or formats
did not match between the images and render target.

TEST=conformance/more/conformance/quickCheckAPI-S_V.html

BUG=angleproject:3384

Change-Id: Iceb9415debfa07436fea1eb4bc23d0ec05679839
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1578957
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>