Add `manifestPeriodCount` to player stats

[tykus160]

Have you read the FAQ and checked for duplicate open issues?
Yes.

Is your feature request related to a problem? Please describe.

For the sake of telemetry, we are trying to monitor how many periods are in our streams. It often help during investigation of production issues.

Describe the solution you'd like

Add manifestPeriodCount to shaka.extern.Stats. For DASH, it should represent number of periods. For other manifests types, it should be probably set explicitly to 1.
When src= mode is used, it should be set to NaN as we are not able to determine it.

Describe alternatives you've considered

Register manifest preprocessor to count it on application side.

Additional context

Are you planning send a PR to add it?
Yes if there's an agreement.

[avelad]

@theodab Can you give your opinion here?

[theodab]

This one also sounds reasonable to me.