[code coverage] Enable parallel fuzzing and increase memory cap.

In https://luci-milo.appspot.com/p/chromium/builders/luci.chromium.ci/linux-code-coverage/1491,
sqlite3_ossfuzz_fuzzer runs out-of-memory (used: 4113Mb; limit: 4096Mb), and kept retrying, so
bump memory cap to 8192Mb because the Linux coverage bot has 204 GB memory.

TBR=inferno, mmoroz, liaoyuke

Change-Id: I65e7065fb5b10f4a6c0f145b57db533498239395
Reviewed-on: https://chromium-review.googlesource.com/c/1378661
Reviewed-by: Shuotao Gao <stgao@chromium.org>
Commit-Queue: Shuotao Gao <stgao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616986}
1 file changed