Implement atob and btoa methods for worker

[prasoon2211]

Moved the Atob and Btoa methods ouside the impl for WorkerMethod trait and made them publicly accessible from WorkerGlobalScopeMethods via proxy methods.

The webidl file already contained the implements line and all I did there was uncomment the line with WorkerGlobalScope implements WindowBase64. I don't know if that does it but running make -f makefile.cargo worked just fine and mach build did not complain about method declaration not being present in the trait definition so I believe it worked fine.

Fixes issue #3238

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/2617

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[highfive]

Thanks for the pull request, and welcome! The Servo team is excited to review your changes, and you should hear from @kmcallister (or someone else) soon.

[jdm]

Solid! If the tests pass, this is ready to merge.