Don't build PFFFT fuzzers' oversized seed corpus

The seed corpus is about 600 MB when zipped. This is probably larger
than useful and contributes the issue of libFuzzer builds taking up
a lot of storage space by adding ~1.2 GB to each (packed) build size
(since the corpus is used twice).

Bug: 952036
Change-Id: I354ec3a37431d3f6c0e83088849203fa4b00fa33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1565309
Commit-Queue: Jonathan Metzman <metzman@chromium.org>
Reviewed-by: Martin Barbella <mbarbella@chromium.org>
Reviewed-by: Max Morin <maxmorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650336}

third_party/pffft/generate_seed_corpus.py

@@ -11,7 +11,7 @@
 import random
 import sys
 
-MAX_INPUT_SIZE = int(1e6)
+MAX_INPUT_SIZE = 5000  # < 1 MB so we don't blow up fuzzer build sizes.
 MAX_FLOAT32 = 3.4028235e+38