Auto merge of #29644 - mrobinson:rename-establishes-containing-block, r=delan

Rename `ComputedValuesExt::establishes_containing_block`

This renames the helper method to be a bit more accurate. For elements with static, relative, and sticky positioning, their containing block is always formed by their nearest block container ancestor. This method is really dealing with style that means an element will establish a containing block for absolutely positioned descendants.

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- [x] These changes do not require tests because they do not change behavior.

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