api_gateway_policy_file no longer works - error

I recently switched to a new AWS account and took a Chalice project with me that was working perfectly - I was using 'api_gateway_policy_file' to point to a file in the '.chalice' directory.

Now with the new AWS login, when I try to deploy, it fails with this error:

`Invalid policy document. Please check the policy syntax and ensure that Principals are valid.`

I manually created a policy and role in the console, pointed the Chalice app to that, and it works fine, however if I copy the valid json into a file, the deploy fails with that error.

Start by reproducing deployment with api_gateway_policy_file pointing to a JSON file in the .chalice directory, using the policy that works when created manually in the AWS console. Compare the file-based deployment path with the console-created policy and verify whether the generated API Gateway policy contains valid Principals. Done means the same valid JSON deploys successfully through Chalice.

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