V8: increase number of shards for arm64 gc stress.

Reason: shard#0 is timing out on swarming.
Expected follow up: if this helps, shard in other
builders as well.

R=hablich@chromium.org
CC=machenbach@chromium.org
BUG=638295

Review-Url: https://codereview.chromium.org/2248363003
2 files changed