Add additional coverage for verifying whether field trials are correctly applied to perfbot runs.

More context on  crbug.com/706414 .

There's varying degree of testing that can be done. Basic coverage would be verifying that the field trials from chrome://version are consistent with command-line flags & testing config.

Although it wouldn't have detected  crbug.com/706414  specifically - since it was about the values of field trial params. We would need to expose which param values are actually being used by Chrome and seeing if those match the testing config.