Use correct proportion when computing the thumb length

Source/platform/scroll/ScrollbarTheme.cpp

 /*
  * Copyright (C) 2011 Apple Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 250 matching lines @@
 int ScrollbarTheme::thumbLength(ScrollbarThemeClient* scrollbar)
 {
     if (!scrollbar->enabled())
         return 0;
 
     float overhang = 0;
     if (scrollbar->currentPos() < 0)
         overhang = -scrollbar->currentPos();
     else if (scrollbar->visibleSize() + scrollbar->currentPos() > scrollbar->totalSize())
         overhang = scrollbar->currentPos() + scrollbar->visibleSize() - scrollbar->totalSize();
-    float proportion = (scrollbar->visibleSize() - overhang) / usedTotalSize(scrollbar);
+    float proportion = 0.0f;
+    float totalSize = usedTotalSize(scrollbar);
+    if (totalSize > 0.0f) {
+        proportion = (scrollbar->visibleSize() - overhang) / totalSize;
+    }
     int trackLen = trackLength(scrollbar);
     int length = round(proportion * trackLen);
     length = std::max(length, minimumThumbLength(scrollbar));
     if (length > trackLen)
         length = 0; // Once the thumb is below the track length, it just goes away (to make more room for the track).
     return length;
 }
 
 int ScrollbarTheme::trackPosition(ScrollbarThemeClient* scrollbar)
 {
@@ skipping 67 matching lines @@
 {
     gMockScrollbarsEnabled = flag;
 }
 
 bool ScrollbarTheme::mockScrollbarsEnabled()
 {
     return gMockScrollbarsEnabled;
 }
 
 }