Port the various WebEvent encoders/decoders to the new IPC serialization format.
<rdar://problem/100901053>
Pull request: https://github.com/WebKit/WebKit/pull/5139
Committed 255441@main (eeb092500c24): <https://commits.webkit.org/255441@main> Reviewed commits have been landed. Closing PR #5139 and removing active labels.