Change the component type of STENCIL_INDEX8 to UNSIGNED_NORMALIZED

That way RenderbufferStorageMultisample accepts STENCIL_INDEX8 as
an internal format for a multisampled buffer (samples > 0) as it
isn't concerned by the restriction on integer component types anymore.
This is consistent with the component type returned by the NVIDIA driver.

BUG=angle:812

Change-Id: Ic03f502ffa082b1011e8127213a5c1df0617ef43
Reviewed-on: https://chromium-review.googlesource.com/227470
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Shannon Woods <shannonwoods@chromium.org>