[Do not land] Add support for experimental Layout Shift Normalized values

[mmocny]

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[mmocny]

@philipwalton This isn't meant to land into master, but I was not sure how to open a PR to compare with master, but to request landing in a separate branch.

Besides the getLSN implementation, I expect the most contentious change was to add support for getting multiple values back via ReportHandler, and the fairly convoluted accompanying change to bindReporter to enable that.

(I haven't added test suite for any of this yet, but did wrote some manual tests and compared to chrome UKM on sample sites manually.)

[philipwalton]

Closing this now the #148 has been merged.