In the spec [1], it only considers text's alignment when computing the `position`. However, the text alignment `start` and `end` can make the same result of what `left` and `right` make, depending on what direction the text uses.
For example, setting text alignment `left` is equal to `start` for the LTR text. Therefore, we could check the result of `ComputedPositionAlign`, which would return correctly direction automatically accoding to the text's base direction.
[1] https://www.w3.org/TR/webvtt1/#cue-computed-position
Differential Revision: https://phabricator.services.mozilla.com/D35272
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extra : moz-landing-system : lando
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