Fixed rounding issue on scrollbar rasterization.

cc/layers/painted_scrollbar_layer.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/painted_scrollbar_layer.h"
 
 #include "base/auto_reset.h"
 #include "base/basictypes.h"
 #include "base/debug/trace_event.h"
 #include "cc/layers/painted_scrollbar_layer_impl.h"
@@ skipping 226 matching lines @@
 UIResourceBitmap PaintedScrollbarLayer::RasterizeScrollbarPart(
     const gfx::Rect& rect,
     ScrollbarPart part) {
   DCHECK(!rect.size().IsEmpty());
 
   SkBitmap skbitmap;
   skbitmap.setConfig(SkBitmap::kARGB_8888_Config, rect.width(), rect.height());
   skbitmap.allocPixels();
 
   SkCanvas skcanvas(skbitmap);
-  skcanvas.translate(SkFloatToScalar(-rect.x()), SkFloatToScalar(-rect.y()));
+
+  // Skia will round to the nearest pixel on scaling, so we need to take that
+  // into account when translating back to the origin.
+  float adjustmentX = round(location_.x() * contents_scale_x());

[danakj, 2014/01/31 18:58:43]

Using location_ here looks wrong, the input |rect| is not always based on
location_, how about when it's the OriginThumbRect()?

[bokan, 2014/01/31 19:00:40]

yikes! You're right, I'll think about this some more...

+  float adjustmentY = round(location_.y() * contents_scale_y());
+  skcanvas.translate(SkFloatToScalar(-adjustmentX),
+      SkFloatToScalar(-adjustmentY));
   skcanvas.scale(SkFloatToScalar(contents_scale_x()),
                  SkFloatToScalar(contents_scale_y()));
 
   gfx::Rect layer_rect = gfx::ScaleToEnclosingRect(
       rect, 1.f / contents_scale_x(), 1.f / contents_scale_y());
   SkRect layer_skrect = RectToSkRect(layer_rect);
   SkPaint paint;
   paint.setAntiAlias(false);
   paint.setXfermodeMode(SkXfermode::kClear_Mode);
   skcanvas.drawRect(layer_skrect, paint);
   skcanvas.clipRect(layer_skrect);
 
   scrollbar_->PaintPart(&skcanvas, part, layer_rect);
   // Make sure that the pixels are no longer mutable to unavoid unnecessary
   // allocation and copying.
   skbitmap.setImmutable();
 
   return UIResourceBitmap(skbitmap);
 }
 
 }  // namespace cc