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/203777Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>