Move Context draw call logic into RendererD3D.

Also move a lot of supporting code.

BUG=angle:789

Change-Id: I098bf7d072ece1f414605783c32ec5354ba63e19
Reviewed-on: https://chromium-review.googlesource.com/226061
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Move Clear logic into RendererD3D.

BUG=angle:789

Change-Id: I56d4398e4c49fe980f649327b105cdba9d944a00
Reviewed-on: https://chromium-review.googlesource.com/226062
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>