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iOS utilizes otool to analyze and distribute test case execution
against x number of shards. This logic was run in the recipe,
such that the sublist of test cases would be passed into the isolate
for execution. Chromium does not support recipe-level sharding.

To mitigate, we clone the otool sharding logic into the test runner.
Each swarming task (shard) is provided with two keys via env vars:
shard index, and total number of shards.

A given test on a given run should be broken down identically across
each swarming task, because the test bundle does not change and the
number of total shards is the same. Because each task also is provided
the index, it can select the corresponding index from the sublist
and execute that sublist of test cases.

There is minimal performance hit, and this logic is only invoked when
those env vars are present, which are only set by swaring for shards
greater than 1.

Change-Id: Id3b4c96cc68ef97b62ed79afc145bbbe55156063
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2040371Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Jeff Yoon <jeffyoon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#739211}