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In this CL, I've extended the functionality of the `generate_perf_sharding`
script to generate the shard_map only for the given benchmark. The user needs
to provide the `benchmark_name` and `builder_name`, and the script will
generate the shard map. Users can also set the number of shards to use as well
as the output file path.

I've also fixed some minor style issues.

Bug: catapult:#
Change-Id: I956ebea0a8d044b1725de4a94eb17f60260a1424
Reviewed-on: https://chromium-review.googlesource.com/1216064Reviewed-by: Ned Nguyen <nednguyen@google.com>
Commit-Queue: Leonard Ge <wangge@google.com>
Cr-Commit-Position: refs/heads/master@{#589974}