Cronvoy: Support for NEL

One of the Cronet Experimental Features is Network Error Logging. These are the parameters:

- "preloaded_report_to_headers"
- Value: a JSON array of preloaded headers, defined below
- Each preloaded header in this array specifies the value of a Report-To header; Cronet will act as if it has already received that header from the specified origin.
- "preloaded_nel_headers"
- Value: a JSON array of preloaded headers, defined below
- Each preloaded header in this array specifies the value of a NEL header; Cronet will act as if it has already received that header from the specified origin.

Each preloaded header is a JSON object with key value pairs as follows:
- "origin"
- Value: string
- The origin that this header should apply to, e.g. "https://example.com"
- "value"
- Value: JSON object or JSON array of JSON objects
- Should contain the value of the Report-To or NEL header for this origin.
- Note that both headers are encoded using JSON on the wire; you should provide the actual JSON object that describes your Reporting or NEL policy, not the string encoding of that JSON object

The Play Store App uses this.