Fix ASSERT failure in XFB test.

With the invariant handling changes, we're also exporting gl_Position
and other builtin variables from the translator where we were not
previously. This was causing an assert in the XFB code, where we
would inadvertently assign a register to gl_Position, and trip up
logic further in the varying handling logic.

BUG=angle:808

Change-Id: I674902da75c572e969f9516033a85c7a7170d7d1
Reviewed-on: https://chromium-review.googlesource.com/225261
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>