Auto merge of #12361 - stshine:blockify-flex-item, r=pcwalton

Blockify elements in a flex container

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The is one of my pull requests that implement basic flexible box layout.
As the spec stated in https://drafts.csswg.org/css-flexbox/#flex-items ,
A inline-level child of a flex container should be converted to its block equivalent, according to CSS2.1 § 9.7.

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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 actually implemented yet.

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