Make scrollbar always have thumb

Bug: 866923
Change-Id: I7e68f4b199ae41280e3745d47849fe3557d24661
Reviewed-on: https://chromium-review.googlesource.com/c/1257723
Commit-Queue: Jinho Bang <jinho.bang@samsung.com>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602222}

third_party/blink/renderer/core/scroll/scrollbar_theme.cc

@@ -291,8 +291,8 @@ int ScrollbarTheme::ThumbLength(const Scrollbar& scrollbar) {
   int length = round(proportion * track_len);
   length = std::max(length, MinimumThumbLength(scrollbar));
   if (length > track_len)
-    length = 0;  // Once the thumb is below the track length, it just goes away
-                 // (to make more room for the track).
+    length = track_len;  // Once the thumb is below the track length,
+                         // it fills the track.
   return length;
 }