Enable overlay scrollbars in Chromium (#14456)

* Enable overlay scrollbars in Chromium

* Test overlay scrollbar migration

* Simplify Chromium overlay scrollbar migration

---------

Co-authored-by: David Heinemeier Hansson <david@hey.com>

migrations/1791376887.sh

@@ -0,0 +1,15 @@
+echo "Enable overlay scrollbars in Chromium-based browsers"
+
+for flags_file in "$HOME"/.config/{chromium,chrome,microsoft-edge-stable,brave,brave-origin}-flags.conf; do
+  [[ -f $flags_file ]] || continue
+  grep -Eq -- '^--enable-features=([^,]+,)*OverlayScrollbar([,:]|$)' "$flags_file" && continue
+
+  if grep -q -- '^--enable-features=' "$flags_file"; then
+    sed -i --follow-symlinks \
+      -e '/^--enable-features=/ s/$/,OverlayScrollbar/' \
+      -e '/^--enable-features=/ s/=,/=/' "$flags_file"
+  else
+    [[ -n $(tail -c1 "$flags_file") ]] && echo >>"$flags_file"
+    echo '--enable-features=OverlayScrollbar' >>"$flags_file"
+  fi
+done