[BEAM-3627] Switch FullWindowedValueCoder to bypass validation

[kennknowles]

I inadvertently left this commit out of #3978 on a bad rebase with master. It is vital for compatibility with some Dataflow hacks that are not so easy to change right away.

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[kennknowles]

run dataflow validatesrunner

[kennknowles]

Since this is code that received LGTM but got messed up when I merged, and it fixes postcommit. Merging TBR @tgroh original reviewer.