Added ability to filter which testers a particular test runs on.
Added ability to filter which testers a particular test runs on.

Currently can filter on:
  - Tester name
  - FYI waterfall vs. main waterfall
  - os_type ("win", "mac")
  - build_config ("Debug", "Release")
  - swarming dimensions

This is a pure refactoring. There are no changes to the autogenerated
chromium.gpu.json or chromium.gpu.fyi.json files. A subsequent CL will
reorganize where a couple of the tests run.

BUG=542370
TBR=dpranke@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/1653963003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#372804}

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