The fast/regions/overflow/overflow-first-and-last-regions.html following layout test is since r180796 (http://trac.webkit.org/changeset/r180796).
This revision updates some test *-expected files after r177774.
The diffs is https://build.webkit.org/results/GTK%20Linux%2064-bit%20Release%20%28Tests%29/r180999%20%286665%29/fast/regions/overflow/overflow-first-and-last-regions-diffs.html
The problem here is that the selected characters are different on GTK and the expected values.
The selected text on GTK is "verflow2_1\nOverflo" and the expected is "erflow2_1\nOverflow". As you can see the difference is one character.
In addition if I modify the selected area x start position very slightly (just 1px) the test passes. I don't know if this could affect to the other ports.
On the other hand I consider that a test showing the selected area text instead of just PASS or FAIL would be more useful.
CSS Regions were removed in Bug 174978.