Auto merge of #12593 - adamncasey:table-row-cell-height, r=pcwalton

Fix table vertical alignment (middle, bottom)

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Fixes table cell vertical alignment (middle, bottom, not yet baseline) when the row contains cells of differing heights.

Moved the work done earlier by @notriddle into a separate public function on TableCellFlow. This function is then called by TableRowFlow once the cell's block size has been calculator.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #12531 (github issue number if applicable).

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- [X] There are tests for these changes

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