Use sized internal formats for unsized depth/stencil textures on ES
3.0 devices that don't advertise the GL_OES_depth_texture extension.

This CL makes the webgl-depth-texture.html conformance test pass on
WebKit's ANGLE backend on iOS.

Bug: angleproject:4591
Change-Id: Ifc45e74258ecf2e6433662bb10b081f1f94bded9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2213678Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>