Cease to depend on undefok for replay mode

In a subsequent CL we are about to remove passing 'undefok' to
fuzztests, since this is no longer needed by the upstream version of
fuzztest. However we were previously using this to alter runtime
behavior of replay mode. Instead, switch to using a buildflag to
determine whether we're acting in a centipede or libfuzzer context.

Bug: 40286211
Cq-Include-Trybots: luci.chromium.try:linux-centipede-asan-rel
Cq-Include-Trybots: luci.chromium.try:linux-libfuzzer-asan-rel
Cq-Include-Trybots: luci.chromium.try:win-libfuzzer-asan-rel
Change-Id: I3ecc382c3a498798c6fe0ce0922b2a48c29442f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5463246


Reviewed-by: Paul Semel <paulsemel@chromium.org>
Commit-Queue: Adrian Taylor <adetaylor@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1289247}