[ios blink] Increase shards to 10

Now that the pool has much more machines (with upgrade
to macOS 15), increase the shard size.

Change-Id: I5ac2f90b3ce7347345de69bc9c08614e71ac3f09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6579817
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Yue She <yueshe@google.com>
Cr-Commit-Position: refs/heads/main@{#1464999}