layout: Do whitespace collapse during breaking and shaping (#31322)

This moves white space collapse to right before breaking and shaping
happens, which is more similar to what happens in legacy layout. This is
the first step toward making this procedure more efficient (avoiding
string copies) and also implementing support for `text-transform`.

Co-authored-by: Rakhi Sharma <atbrakhi@igalia.com>

components/layout_2020/flexbox/construct.rs

@@ -153,7 +153,6 @@ where
                             (&run.info).into(),
                             run.info.style,
                             run.text.into(),
-                            false, /* has_uncollapsible_content */
                         )
                     });
                     let bfc = BlockFormattingContext::construct_for_text_runs(