Define libwebp max allocation limit in GN fuzzer targets

This upper bound of 800MiB is previously enforced in the oss-fuzz
targets [1], so we are adding it here too for consistency.

[1]: github.com/google/oss-fuzz/blob/master/projects/libwebp/build.sh#L19

Bug: 901042
Change-Id: I5d1f3a970340c0c60c39e52ab2f959e0253ff63c
Reviewed-on: https://chromium-review.googlesource.com/c/1365655
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Reviewed-by: James Zern <jzern@google.com>
Commit-Queue: Madeleine Barowsky <mbarowsky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614611}
1 file changed