Previously removed supporting code for the workaround
"dontRemoveInvariantForFragmentInput" but missed this declaration.

Bug: angleproject:3285
Change-Id: Ia23b424f2ccbdfe98b7cec79e4290b3621e44238
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1601945Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Tobin Ehlis <tobine@google.com>