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Change the model that SmartDimModelV3 flag uses.
This flag is not enabled via Finch for any users yet.

* Add 20190521 preprocessor .pb file
* Remove 20190221 preprocessor .pb file

	flag through chrome://flags, and manually invoked Smart Dim via
	its D-Bus interface.

Bug: chromium:937063
Change-Id: Iac3a1613c0d3ce2af830e6eedda1cfa13eec9d31
Tested: Pushed updated Chrome to DUT, enabled the SmartDimV3 feature
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1813538
Auto-Submit: Andrew Moylan <amoylan@chromium.org>
Commit-Queue: calamity <calamity@chromium.org>
Reviewed-by: Jia Meng <jiameng@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698378}