When calling Framebuffer::commitWebGL1DepthStencilIfConsistent with
textures attached to depth or stencil, an invalid ImageIndex would be
provided and later cause crashes when trying to index image arrays with a
-1 mip level.

BUG=angleproject:1708

Change-Id: Iadd159ad740aa79561de823d8812c6b07454e5e5
Reviewed-on: https://chromium-review.googlesource.com/456840
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>