Auto merge of #12382 - stshine:anonymous-block, r=SimonSapin

layout: Clear non-inherited properties on anonymous block

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This is one of my pull requests that imlement basic flexible box layout.
This pull request adds a `modify_style_for_anonymous_flow()` function to use initial values for non-inherited properties and parent values for inherited properties as the flow style. It also set border and outline to zero and set the display property from the parameter. This function may also be reused to modify style of anonymous table object in the future.
Part of code comes from `cascade_anonymous()` from @SimonSapin .

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- [ ] These changes fix #__ (github issue number if applicable).

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