Avoid div-by-zero in thumb quad computation

cc/layers/scrollbar_layer_impl_base.cc

 // Copyright 2013 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 "cc/layers/scrollbar_layer_impl_base.h"
 
 #include <algorithm>
 #include "cc/trees/layer_tree_impl.h"
 #include "ui/gfx/rect_conversions.h"
 
@@ skipping 194 matching lines @@
   // means the quads may extend outside the layer's bounds.
 
   // With the length known, we can compute the thumb's position.
   float track_length = TrackLength();
   int thumb_length = ThumbLength();
   int thumb_thickness = ThumbThickness();
 
   // With the length known, we can compute the thumb's position.
   float clamped_current_pos =
       std::min(std::max(current_pos_, 0.f), static_cast<float>(maximum_));
-  float ratio = clamped_current_pos / maximum_;
-  float max_offset = track_length - thumb_length;
-  int thumb_offset = static_cast<int>(ratio * max_offset) + TrackStart();
+
+  int thumb_offset = TrackStart();
+  if (maximum_ > 0) {
+    float ratio = clamped_current_pos / maximum_;
+    float max_offset = track_length - thumb_length;
+    thumb_offset += static_cast<int>(ratio * max_offset);
+  }
 
   float thumb_thickness_adjustment =
       thumb_thickness * (1.f - thumb_thickness_scale_factor_);
 
   gfx::RectF thumb_rect;
   if (orientation_ == HORIZONTAL) {
     thumb_rect = gfx::RectF(thumb_offset,
                             vertical_adjust_ + thumb_thickness_adjustment,
                             thumb_length,
                             thumb_thickness - thumb_thickness_adjustment);
@@ skipping 11 matching lines @@
 }
 
 void ScrollbarLayerImplBase::ScrollbarParametersDidChange() {
   if (!clip_layer_ || !scroll_layer_)
     return;
 
   scroll_layer_->SetScrollbarPosition(this, clip_layer_);
 }
 
 }  // namespace cc