[Flight] Don't increase serializedSize for every recursive pass

[sebmarkbage]

I noticed that we increase this in the recursive part of the algorithm. This would mean that we'd count a key more than once if it has Server Components inside it recursively resolving. This moves it out to where we enter from toJSON. Which is called once per JSON entry (and therefore once per key).

[react-sizebot]

Comparing: 0ca8420...63042a5

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.js	=	6.68 kB	6.68 kB	+0.05%	1.83 kB	1.83 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	528.27 kB	528.27 kB	=	93.14 kB	93.14 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.69 kB	6.69 kB	+0.05%	1.83 kB	1.83 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	633.90 kB	633.90 kB	=	111.33 kB	111.33 kB
facebook-www/ReactDOM-prod.classic.js	=	671.68 kB	671.68 kB	=	117.77 kB	117.77 kB
facebook-www/ReactDOM-prod.modern.js	=	661.96 kB	661.96 kB	=	116.21 kB	116.21 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 63042a5