If the same level of a texture is attached to multiple color
attachments of a framebuffer object, for example, the WebGL
conformance tests require generating a GL_FRAMEBUFFER_UNSUPPORTED
error. The Direct3D backend already had this restriction; apply it to
all backends when the WebGL compatibility extension is enabled.

Fixes the following WebGL conformance tests with the pass-through
command decoder in Chrome:
  conformance/extensions/webgl-draw-buffers-framebuffer-unsupported
  conformance2/rendering/framebuffer-unsupported

BUG=angleproject:2168

Change-Id: I340d06ca0ee969989c6c5725512b1b9542281477
Reviewed-on: https://chromium-review.googlesource.com/699856
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>