Add a Canvas::SaveAndUnscale() method for pixel-accurate drawing.

ash/frame/default_header_painter.cc

Index: ash/frame/default_header_painter.cc
diff --git a/ash/frame/default_header_painter.cc b/ash/frame/default_header_painter.cc
index a80a3c6bc0ced68a0adaa13104d5ca2c4a571fbf..ab9dd1806f17940a428f9d920d350e5048d10085 100644
--- a/ash/frame/default_header_painter.cc
+++ b/ash/frame/default_header_painter.cc
@@ -262,18 +262,13 @@ void DefaultHeaderPainter::PaintTitleBar(gfx::Canvas* canvas) {
 }
 
 void DefaultHeaderPainter::PaintHeaderContentSeparator(gfx::Canvas* canvas) {
-  SkColor color = (mode_ == MODE_ACTIVE) ?
-      kHeaderContentSeparatorColor :
-      kHeaderContentSeparatorInactiveColor;
-
+  const float scale = canvas->SaveAndUnscale();
+  gfx::RectF rect(0, painted_height_ * scale - 1, view_->width() * scale, 1);
   SkPaint paint;
-  paint.setColor(color);
-  // Draw the line as 1px thick regardless of scale factor.
-  paint.setStrokeWidth(0);
-
-  float thickness = 1 / canvas->image_scale();
-  SkScalar y = SkIntToScalar(painted_height_) - SkFloatToScalar(thickness);
-  canvas->sk_canvas()->drawLine(0, y, SkIntToScalar(view_->width()), y, paint);
+  paint.setColor((mode_ == MODE_ACTIVE) ?
+      kHeaderContentSeparatorColor : kHeaderContentSeparatorInactiveColor);
+  canvas->sk_canvas()->drawRect(gfx::RectFToSkRect(rect), paint);
+  canvas->Restore();
 }
 
 void DefaultHeaderPainter::LayoutLeftHeaderView() {