Draw infobar arrow border with width of 1px on any scale factor.

chrome/browser/ui/views/infobars/infobar_background.cc

Index: chrome/browser/ui/views/infobars/infobar_background.cc
diff --git a/chrome/browser/ui/views/infobars/infobar_background.cc b/chrome/browser/ui/views/infobars/infobar_background.cc
index 7e9fe105f810af8ee28893f3064b0fc34bf69ece..2bd8da7d21763429c95df7292cfc2d26b3b1b3c9 100644
--- a/chrome/browser/ui/views/infobars/infobar_background.cc
+++ b/chrome/browser/ui/views/infobars/infobar_background.cc
@@ -8,6 +8,7 @@
 #include "chrome/browser/ui/views/infobars/infobar_view.h"
 #include "components/infobars/core/infobar.h"
 #include "third_party/skia/include/effects/SkGradientShader.h"
+#include "ui/base/material_design/material_design_controller.h"
 #include "ui/gfx/canvas.h"
 #include "ui/gfx/color_utils.h"
 #include "ui/views/view.h"
@@ -30,20 +31,22 @@ void InfoBarBackground::Paint(gfx::Canvas* canvas, views::View* view) const {
   gradient_points[1].iset(0, view->height());
   SkColor gradient_colors[2] = { top_color_, bottom_color_ };
   SkPaint paint;
-  paint.setStrokeWidth(
-      SkIntToScalar(InfoBarContainerDelegate::kSeparatorLineHeight));
   paint.setStyle(SkPaint::kFill_Style);
-  paint.setStrokeCap(SkPaint::kRound_Cap);
   paint.setShader(SkGradientShader::MakeLinear(
-      gradient_points, gradient_colors, NULL, 2, SkShader::kClamp_TileMode));
+      gradient_points, gradient_colors, nullptr, 2, SkShader::kClamp_TileMode));
 
   InfoBarView* infobar = static_cast<InfoBarView*>(view);
   SkCanvas* canvas_skia = canvas->sk_canvas();
   canvas_skia->drawPath(infobar->fill_path(), paint);
 
-  paint.setShader(NULL);
+  paint.setShader(nullptr);

[Peter Kasting, 2016/03/23 00:28:17]

Nit: Rather than reset this, let's just use two different SkPaints for the
stroke and the fill.

   paint.setColor(SkColorSetA(separator_color_,
                              SkColorGetA(gradient_colors[0])));
+  paint.setStrokeCap(SkPaint::kRound_Cap);

[Peter Kasting, 2016/03/22 23:39:18]

Why do we need to set the cap for a hairline?

+  paint.setStrokeWidth(
+      ui::MaterialDesignController::IsModeMaterial()
+          ? 0
+          : SkIntToScalar(InfoBarContainerDelegate::kSeparatorLineHeight));
   paint.setStyle(SkPaint::kStroke_Style);
   // Anti-alias the path so it doesn't look goofy when the edges are not at 45
   // degree angles, but don't anti-alias anything else, especially not the fill,
@@ -53,9 +56,7 @@ void InfoBarBackground::Paint(gfx::Canvas* canvas, views::View* view) const {
   paint.setAntiAlias(false);
 
   // Now draw the separator at the bottom.
-  canvas->FillRect(
-      gfx::Rect(0,
-                view->height() - InfoBarContainerDelegate::kSeparatorLineHeight,
-                view->width(), InfoBarContainerDelegate::kSeparatorLineHeight),
-      separator_color_);
+  paint.setColor(separator_color_);
+  SkScalar bottom = SkIntToScalar(view->height()) - SK_ScalarHalf;

[Peter Kasting, 2016/03/23 00:28:17]

This seems wrong at >1x.  It really wants to move up by 0.5 unscaled pixels. 
(similar to the issue with defining the stroke path.)  Since antialiasing is
off, we won't get a partial line across 2 vertical pixels; instead we'll get a
line that might be off by 1 pixel depending on how the non-antialiased drawing
path works.  (At >2x this is presumably even more likely to be incorrect.)

[Evan Stade, 2016/03/23 00:35:46]

You're right, it does have bugs at >2x scales. I'll look into this more.

+  canvas_skia->drawLine(0, bottom, SkIntToScalar(view->width()), bottom, paint);
 }