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Currently, most feature validation happens in Parse(). Add a Validate()
function that can be called after Parse() so that if a feature is
generated, validation can still be performed (independent of whether or
not the feature is created from a JSON value).  In the future, these
validations should be performed as part of the compile process, but this
is a starting point.

Also update tests that parsed features and used JSONFeatureProvider only
to test this validation. This way, once we move away from
JSON-based features and JSONFeatureProvider, these tests are still
active.

BUG=280286

Review-Url: https://codereview.chromium.org/2150193002
Cr-Commit-Position: refs/heads/master@{#406076}