Guard against division by zero for useless scroll bars.

ui/views/controls/scrollbar/base_scroll_bar.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/views/controls/scrollbar/base_scroll_bar.h"
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/callback.h"
 #include "base/compiler_specific.h"
@@ skipping 395 matching lines @@
     content_size = 0;
   if (contents_scroll_offset < 0)
     contents_scroll_offset = 0;
   if (contents_scroll_offset > content_size)
     contents_scroll_offset = content_size;
   contents_scroll_offset_ = contents_scroll_offset;
 
   // Thumb Height and Thumb Pos.
   // The height of the thumb is the ratio of the Viewport height to the
   // content size multiplied by the height of the thumb track.
-  double ratio = static_cast<double>(viewport_size) / contents_size_;
+  double ratio =
+      std::min(1.0, static_cast<double>(viewport_size) / contents_size_);
   int thumb_size = static_cast<int>(ratio * GetTrackSize());
   thumb_->SetSize(thumb_size);
 
   int thumb_position = CalculateThumbPosition(contents_scroll_offset);
   thumb_->SetPosition(thumb_position);
 }
 
 int BaseScrollBar::GetPosition() const {
   return thumb_->GetPosition();
 }
@@ skipping 71 matching lines @@
   int thumb_max = GetTrackSize() - thumb_->GetSize();
   if (contents_scroll_offset + viewport_size_ == contents_size_)
     return thumb_max;
   return (contents_scroll_offset * thumb_max) /
          (contents_size_ - viewport_size_);
 }
 
 int BaseScrollBar::CalculateContentsOffset(int thumb_position,
                                            bool scroll_to_middle) const {
   int thumb_size = thumb_->GetSize();
+  int track_size = GetTrackSize();
+  if (track_size == thumb_size)
+    return 0;
   if (scroll_to_middle)
     thumb_position = thumb_position - (thumb_size / 2);
   return (thumb_position * (contents_size_ - viewport_size_)) /
-         (GetTrackSize() - thumb_size);
+         (track_size - thumb_size);
 }
 
 void BaseScrollBar::SetThumbTrackState(CustomButton::ButtonState state) {
   thumb_track_state_ = state;
   SchedulePaint();
 }
 
 }  // namespace views