This property is used by approximately 55% of page loads.
To implement the line breaking behavior, the "breaking strategy" has
been cleaned up and abstracted. This should allow us to easily support
other similar properties in the future, such as `text-overflow` and
`word-break`.
This exposed some problems in our clipping logic, which was never
properly rewritten for the stacking context reform. The clipping code
worked in terms of a stack of clips, but the new stacking context code
has no concept of a stack of clip regions. Fixing that in turn exposed
some flaky/incorrect tests:
* `borders` had an incorrect reference image, as far as I can tell.
* `negative_margins` had some stray pixels, fixed by changing the text.
