Fix link error when using varyings with "dx_". Varyings beginning with "dx_" would not get decorated properly, or at all, which could cause potential internal variables and certainly caused link errors. Fix this by no longer treating the "dx_" prefix differently. Also fix our naming of "dx_Position" to be consistent with our other internal types. BUG=angle:678 Change-Id: I03da0494a3d934d82ae7b3f8f12a576ff9bc3869 Reviewed-on: https://chromium-review.googlesource.com/203777 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>