Make the header/content separator into a hairline for apps in hidpi

ash/frame/header_painter_util.cc

Index: ash/frame/header_painter_util.cc
diff --git a/ash/frame/header_painter_util.cc b/ash/frame/header_painter_util.cc
index 77123e6f4f8d2f93af26b03324dbb49139e6c265..ca838248ec5b96b6eb1a4a23ee4392df92ed034e 100644
--- a/ash/frame/header_painter_util.cc
+++ b/ash/frame/header_painter_util.cc
@@ -74,10 +74,9 @@ gfx::Rect HeaderPainterUtil::GetTitleBounds(
   int x = icon ?
       icon->bounds().right() + kTitleIconOffsetX : kTitleNoIconOffsetX;
   int height = title_font_list.GetHeight();
-  int y = std::max(
-      0,
-      static_cast<int>(std::ceil(
-          (caption_button_container->height() - height) / 2.0f)));
+  // Floor when computing the center of |caption_button_container| and when
+  // computing the center of the text.
+  int y = std::max(0, (caption_button_container->height() / 2) - (height / 2));
   int width = std::max(
       0, caption_button_container->x() - kTitleCaptionButtonSpacing - x);
   return gfx::Rect(x, y, width, height);