Instead of always compiling D3D11 shaders with shader model 4, compile
with the latest profile that the current feature level allows.

BUG=angle:495

Change-Id: I478d3d4a1aadf24d893508eee1a8c11e501bc084
Reviewed-on: https://chromium-review.googlesource.com/190340Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>