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

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

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/ui/views/infobars/infobar_background.h"
 
 #include "chrome/browser/ui/infobar_container_delegate.h"
 #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"
 
 InfoBarBackground::InfoBarBackground(
     infobars::InfoBarDelegate::Type infobar_type)
     : separator_color_(SK_ColorBLACK),
       top_color_(infobars::InfoBar::GetTopColor(infobar_type)),
       bottom_color_(infobars::InfoBar::GetBottomColor(infobar_type)) {
   SetNativeControlColor(
       color_utils::AlphaBlend(top_color_, bottom_color_, 128));
 }
 
 InfoBarBackground::~InfoBarBackground() {
 }
 
 void InfoBarBackground::Paint(gfx::Canvas* canvas, views::View* view) const {
   SkPoint gradient_points[2];
   gradient_points[0].iset(0, 0);
   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,
   // lest we get weird color bleeding problems.
   paint.setAntiAlias(true);
   canvas_skia->drawPath(infobar->stroke_path(), paint);
   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);
 }