Pass 0 for the sample count to RenderTarget when there is no multisampling.

BUG=angle:520

Change-Id: Idc9a19ece620d107b87110dac8e1ef7a24f565cc
Reviewed-on: https://chromium-review.googlesource.com/228551
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>