Fixed rounding issue on scrollbar rasterization.

cc/layers/painted_scrollbar_layer.cc

Index: cc/layers/painted_scrollbar_layer.cc
diff --git a/cc/layers/painted_scrollbar_layer.cc b/cc/layers/painted_scrollbar_layer.cc
index 71a37d48883f8f07d592ff561e288e344917ed65..ae8efd00bdbbf11c9b77b92ad73a794bd1b02b31 100644
--- a/cc/layers/painted_scrollbar_layer.cc
+++ b/cc/layers/painted_scrollbar_layer.cc
@@ -169,7 +169,7 @@ gfx::Rect PaintedScrollbarLayer::ScrollbarLayerRectToContentRect(
     const gfx::Rect& layer_rect) const {
   // Don't intersect with the bounds as in LayerRectToContentRect() because
   // layer_rect here might be in coordinates of the containing layer.
-  gfx::Rect expanded_rect = gfx::ScaleToEnclosingRect(
+  gfx::Rect expanded_rect = gfx::ScaleToEnclosedRect(

[danakj, 2014/02/10 19:17:28]

I don't believe this is correct.

Say you have a scale of 1.5 and a width of 11. Then your content bounds width
will be 17.

If you take the enclosed here, you'll get only 16, so your width won't fill the
entire content_bounds().

This is why we clamp to the content_bounds() after doing Enclosing on line 176.

       layer_rect, contents_scale_y(), contents_scale_y());
   // We should never return a rect bigger than the content_bounds().
   gfx::Size clamped_size = expanded_rect.size();
@@ -248,6 +248,8 @@ UIResourceBitmap PaintedScrollbarLayer::RasterizeScrollbarPart(
   skcanvas.scale(SkFloatToScalar(contents_scale_x()),
                  SkFloatToScalar(contents_scale_y()));
 
+  // Since rect is an enclosed rect around the scaled layer_rect, we must take
+  // the enclosing rect when unscaling so we don't lose a pixel in truncation.
   gfx::Rect layer_rect = gfx::ScaleToEnclosingRect(

[danakj, 2014/02/10 19:17:28]

It seems like this one is the one we should switch to enclosed? Or maybe you can
explain what I'm missing there?

Maybe we need to scale the skcanvas to the |rect| instead of using the
contents_scale_x/y directly?

[danakj, 2014/02/10 19:19:37]

A possible nice change would be to pass in the layer rect instead of having to
do this math here again, since the caller already had the layer rect.

[bokan, 2014/02/11 00:01:53]

If the one above is enclosed then it can be clipped which means that layer_rect
* contents_scale_x != rect which is assumed here. I've changed to pass in the
layer rect as you suggested. 

       rect, 1.f / contents_scale_x(), 1.f / contents_scale_y());
   SkRect layer_skrect = RectToSkRect(layer_rect);