Overlay scrollbars expand only when mouse is near thumb

cc/layers/scrollbar_layer_impl_base.cc

Index: cc/layers/scrollbar_layer_impl_base.cc
diff --git a/cc/layers/scrollbar_layer_impl_base.cc b/cc/layers/scrollbar_layer_impl_base.cc
index 839c0b7f927d488f9ce76813ebd40b7abf66569f..06a91c88e793fa1d4db3529e6c002fa752282a0a 100644
--- a/cc/layers/scrollbar_layer_impl_base.cc
+++ b/cc/layers/scrollbar_layer_impl_base.cc
@@ -107,6 +107,37 @@ bool ScrollbarLayerImplBase::SetThumbThicknessScaleFactor(float factor) {
   return true;
 }
 
+gfx::Rect ScrollbarLayerImplBase::ComputeExpandedThumbQuadRect() const {
+  DCHECK(is_overlay_scrollbar());
+  float track_length = TrackLength();
+  int thumb_length = ThumbLength();
+  int thumb_thickness = ThumbThickness();
+  float maximum = scroll_layer_length_ - clip_layer_length_;
+
+  // With the length known, we can compute the thumb's position.
+  float clamped_current_pos = std::min(std::max(current_pos_, 0.f), maximum);
+
+  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);
+  }
+
+  gfx::RectF thumb_rect;
+  if (orientation_ == HORIZONTAL) {
+    thumb_rect = gfx::RectF(thumb_offset, vertical_adjust_, thumb_length,
+                            thumb_thickness);
+  } else {
+    thumb_rect = gfx::RectF(is_left_side_vertical_scrollbar_
+                                ? bounds().width() - thumb_thickness
+                                : 0,
+                            thumb_offset, thumb_thickness, thumb_length);
+  }
+
+  return gfx::ToEnclosingRect(thumb_rect);
+}
+
 gfx::Rect ScrollbarLayerImplBase::ComputeThumbQuadRect() const {
   // Thumb extent is the length of the thumb in the scrolling direction, thumb
   // thickness is in the perpendicular direction. Here's an example of a