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..d1fd6b23c7513334a051a822b64cf2f98c8272be 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,

[danakj, 2016/01/12 20:07:03]

"zoom" isn't a concept here. Instead "If device scale is applied during painting
instead of by the compositor..."

[oshima, 2016/01/12 23:19:05]

Done.

+  // 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::Size scaled_layer_size =
+      gfx::ScaleToFlooredSize(layer_rect.size(), scale, scale);

[danakj, 2016/01/12 20:07:03]

But why isn't blink just making the scrollbar layer the right size?

[oshima, 2016/01/12 23:19:05]

It does (in viewport size) and that's why I have to convert it back to DIP so
that painter draw scaled image, not bigger one.

That's being said, I found the issue in current approach where it can't handle
the scale factor change properly.

Please hold, and will send request when it is resolved.

+
   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>(scaled_layer_size.width());
   float scale_y =
-      content_rect.height() / static_cast<float>(layer_rect.height());
+      content_rect.height() / static_cast<float>(scaled_layer_size.height());
 
   skcanvas.scale(SkFloatToScalar(scale_x),
                  SkFloatToScalar(scale_y));
   skcanvas.translate(SkFloatToScalar(-layer_rect.x()),
                      SkFloatToScalar(-layer_rect.y()));
 
-  SkRect layer_skrect = RectToSkRect(layer_rect);
+  gfx::Rect scaled_layer_rect(layer_rect.origin(), scaled_layer_size);
+  SkRect layer_skrect = RectToSkRect(scaled_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, scaled_layer_rect);
   // Make sure that the pixels are no longer mutable to unavoid unnecessary
   // allocation and copying.
   skbitmap.setImmutable();