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This CL refactors the model to more closely match the data. In particular,
the model used to be colors[mode][name] and is now colors[name][mode],
which matches the data. This is in preparation for implementing
opacities, a 'generate-dark-mode-only' mode, and dark-mode-agnostic
properties.

Other minor changes:
- BaseGenerator.colors is now BaseGenerator.model[VariableType.COLOR]
- ModeVariables now has methods that let it be used more like a dict()

Bug: 1018654
Change-Id: I5422178053434b49ac64a6290983092cbbbf0ab0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2245968
Commit-Queue: calamity <calamity@chromium.org>
Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780201}