Enable platform views for Flutter Gallery on iOS. (#29632)
Enable platform views for Flutter Gallery on iOS. (#29632)

This results in running Flutter Gallery with one less thread (platform
and gpu threads are the the same).

This is very likely to cause a regression in the iOS Gallery benchmarks.
I'm mainly interested in landing this to see how much the benchmarks
regress, and it's likely that we will revert it shortly after landing.
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