refactor: use public accessors for network records

[patrickhulce]

Brought over from discussion

The two biggest holdouts are _resourceType and _initiator which are functions and not getter properties.

[brendankenny]

nice, though this does move the problem mentioned in #1513 (comment) to the tests, so if the implementation of the getters change the test fixtures will presumably have to be updated.

Not sure how much of a danger that is, though, and I'd say simulating the getters in the test fixtures is definitely better than accessing the internal properties in the actual gatherers/audits...

[brendankenny]

after discussion today, sounds like the solution is this PR + storing performance logs (or whatever format) for network activity for tests. Tests would use something like NetworkRecorder.recordsFromLogs() to convert them into the networkRecords format gatherers and audits expect, and we can drop all use of the internal variable names for tests.

Main things we'd need to establish would be an easy format to create for creating test files and (if we don't end up using the performance logs we currently support) an easy function for conversion to networkRecords

[paulirish]

honestly we're not in a big rush here. :)
let's drop this for now.