Auto merge of #12371 - stshine:flex-block-flow, r=notriddle

Treat flex flow as block in float calculation

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This is on of my pull requests that implement basic flexible box layout.
Currently call `is_block_like()` on flex flow returns false. This would fix some errors in tests, for example 'flexbox-justify-content-horiz-001b.htm'.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because flexbox is not implemented yet.

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