CreateButtonBackground() has been creating unpremultiplied bitmaps.

ui/gfx/skbitmap_operations.cc

Index: ui/gfx/skbitmap_operations.cc
diff --git a/ui/gfx/skbitmap_operations.cc b/ui/gfx/skbitmap_operations.cc
index 1535b7032d390f20ac831ab83fa41938e4cca787..1d810508ca629d109efc041a94bff9c1c0c60b0f 100644
--- a/ui/gfx/skbitmap_operations.cc
+++ b/ui/gfx/skbitmap_operations.cc
@@ -129,16 +129,24 @@ SkBitmap SkBitmapOperations::CreateMaskedBitmap(const SkBitmap& rgb,
 SkBitmap SkBitmapOperations::CreateButtonBackground(SkColor color,
                                                     const SkBitmap& image,
                                                     const SkBitmap& mask) {
+  // TODO: can we just do this with a couple SkCanvas draw calls?

[danakj, 2016/06/22 00:51:27]

TODO format is:

TODO(chromiumname): text
or
TODO(crbug.com/LMNOP): text

[mtklein_C, 2016/07/28 19:56:04]

Gotcha.  Good point... no one will probably ever care to fix these.

+  // e.g. drawColor(color) -> drawBitmap(image) -> drawBitmap(mask, kDstIn_Mode)
+  // This requires we first pin down the premul/unpremul state of each bitmap.
+
+  // TODO: It sure seems like image is actually unpremultiplied.
+  // The math producing dst_row[x] below is a correct SrcOver when bg is

[mtklein_C, 2016/06/22 15:19:40]

Is this one too far away?

[sky, 2016/06/22 15:37:43]

It wasn't obvious to me that the two relate. Comments with TODO in them are easy
to ignore.

[mtklein_C, 2016/07/28 19:56:04]

Fixed up, I think.

+  // premultiplied and img is unpremultiplied.
   DCHECK(image.colorType() == kN32_SkColorType);
   DCHECK(mask.colorType() == kN32_SkColorType);
 
   SkBitmap background;
   background.allocN32Pixels(mask.width(), mask.height());
 
+  // We premultiply bg here.

[sky, 2016/06/22 15:17:40]

This comment just documents the code, and isn't very helpful. A comment that
describes why we need to premultiply helps those looking at the code understand
why the code is the way it is.

[mtklein_C, 2016/07/28 19:56:04]

Gone.

   double bg_a = SkColorGetA(color);
-  double bg_r = SkColorGetR(color);
-  double bg_g = SkColorGetG(color);
-  double bg_b = SkColorGetB(color);
+  double bg_r = SkColorGetR(color) * (bg_a / 255.0);
+  double bg_g = SkColorGetG(color) * (bg_a / 255.0);
+  double bg_b = SkColorGetB(color) * (bg_a / 255.0);
 
   SkAutoLockPixels lock_mask(mask);
   SkAutoLockPixels lock_image(image);
@@ -157,12 +165,13 @@ SkBitmap SkBitmapOperations::CreateButtonBackground(SkColor color,
       double img_g = SkColorGetG(image_pixel);
       double img_b = SkColorGetB(image_pixel);
 
-      double img_alpha = static_cast<double>(img_a) / 255.0;
+      double img_alpha = img_a / 255.0;
       double img_inv = 1 - img_alpha;
 
       double mask_a = static_cast<double>(SkColorGetA(mask_row[x])) / 255.0;
 
       dst_row[x] = SkColorSetARGB(
+          // TODO: why not the usual SrcOver alpha?
           static_cast<int>(std::min(255.0, bg_a + img_a) * mask_a),
           static_cast<int>(((bg_r * img_inv) + (img_r * img_alpha)) * mask_a),
           static_cast<int>(((bg_g * img_inv) + (img_g * img_alpha)) * mask_a),