Fix alwaysThrottleDisappearingFallbacks

[rickhanlonii]

For alwaysThrottleDisappearingFallbacks, we need to update the workloop to throttle when the root completes. This fixes the bug found in #28629.

[react-sizebot]

Comparing: 5a75f9e...fc72ca9

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.min.js	=	176.93 kB	176.93 kB	=	54.95 kB	54.95 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	173.36 kB	173.36 kB	=	54.08 kB	54.08 kB
facebook-www/ReactDOM-prod.classic.js	+0.01%	595.27 kB	595.36 kB	+0.02%	104.55 kB	104.57 kB
facebook-www/ReactDOM-prod.modern.js	+0.02%	578.53 kB	578.62 kB	+0.02%	101.57 kB	101.59 kB
test_utils/ReactAllWarnings.js	Deleted	65.49 kB	0.00 kB	Deleted	16.16 kB	0.00 kB

Significant size changes

Includes any change greater than 0.2%:

Expand to show

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
test_utils/ReactAllWarnings.js	Deleted	65.49 kB	0.00 kB	Deleted	16.16 kB	0.00 kB

Generated by 🚫 dangerJS against fc72ca9

[rickhanlonii]

Closed by #28639