Scale scrollbar in use-zoom-for-dsf mode

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 13826e7e462ade4d5d9743a98f3eaa0c3462a7ea..452351ea6f602ec1abd43c4d50a3dd7ee26553d2 100644
--- a/cc/layers/painted_scrollbar_layer.cc
+++ b/cc/layers/painted_scrollbar_layer.cc
@@ -279,28 +279,36 @@ UIResourceBitmap PaintedScrollbarLayer::RasterizeScrollbarPart(
   DCHECK(!content_rect.size().IsEmpty());
   DCHECK(!layer_rect.size().IsEmpty());
 
+  // If zoom is used to scale the content for device scale factor,
+  // convert the size of layer_rect to DIP so that it will generate
+  // the image for scaled scrollbar.
+  float scale = 1.0f / layer_tree_host()->painted_device_scale_factor();
+  gfx::Rect adjusted_layer_rect(

[skobes, 2016/01/07 21:49:10]

"scaled_layer_rect" is clearer

[oshima, 2016/01/09 00:58:39]

I used adjusted because its origin isn't scaled. I changed it to scaled anyway,
but let me know if you're convinced.

+      layer_rect.origin(),

[skobes, 2016/01/07 21:49:10]

Don't we need to scale the origin as well?  I assume origin and size are in the
same coordinate system...

[oshima, 2016/01/09 00:58:39]

This is correct. We want to scale the size only and the drawing code adjust the
canvas's coordinate so that the image will be drawn at 0.0 in the SkBitmap. I
updated a bit to make it clearer. 

+      gfx::ScaleToFlooredSize(layer_rect.size(), scale, scale));
+
   SkBitmap skbitmap;
   skbitmap.allocN32Pixels(content_rect.width(), content_rect.height());
   SkCanvas skcanvas(skbitmap);
 
   float scale_x =
-      content_rect.width() / static_cast<float>(layer_rect.width());
+      content_rect.width() / static_cast<float>(adjusted_layer_rect.width());
   float scale_y =
-      content_rect.height() / static_cast<float>(layer_rect.height());
+      content_rect.height() / static_cast<float>(adjusted_layer_rect.height());
 
   skcanvas.scale(SkFloatToScalar(scale_x),
                  SkFloatToScalar(scale_y));
-  skcanvas.translate(SkFloatToScalar(-layer_rect.x()),
-                     SkFloatToScalar(-layer_rect.y()));
+  skcanvas.translate(SkFloatToScalar(-adjusted_layer_rect.x()),
+                     SkFloatToScalar(-adjusted_layer_rect.y()));
 
-  SkRect layer_skrect = RectToSkRect(layer_rect);
+  SkRect layer_skrect = RectToSkRect(adjusted_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);
+  scrollbar_->PaintPart(&skcanvas, part, adjusted_layer_rect);
   // Make sure that the pixels are no longer mutable to unavoid unnecessary
   // allocation and copying.
   skbitmap.setImmutable();