[task scheduler] Get the isolate OsType from dimensions
[task scheduler] Get the isolate OsType from dimensions

This allows us to pick up optional isolates, eg. ios_bin.

BUG=skia:

Change-Id: Id25de16a00a0499dacf258af40378457cbf4fee2
Reviewed-on: https://skia-review.googlesource.com/4585
Reviewed-by: Ben Wagner <3ef7217be91069877d94f7907ce5479000772cd3@google.com>
Commit-Queue: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>

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