Previously GL_NONE were being used, causing
nativegl::SupportsNativeRendering to think the bound texture wasn't
renderable, and TextureGL::copySubTextureHelper (called via
CopyTexSubImageCHROMIUM) to use CPU readback. This affected the WebGL
DrawingBuffer's premultiplyAlpha:false code path.

Thanks to geofflang@ for the patch.

Bug: chromium:1136613
Change-Id: Iee650156983f880fa802b3a17d3989f5a9322e91
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2472535Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>