The 3.2 spec and the dEQP tests are quite clear in only requiring
the 'flat' on fragment shader inputs. This restriction remains on
older shader versions.

Bug: angleproject:5409
Change-Id: Icc4b113b404280c2867a3e0bf6f77256a4316bff
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2574826
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>