Our masker didn't account for the source image also having alpha....

skia/ext/image_operations.cc

 // Copyright (c) 2006-2008 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.
 //
 #define _USE_MATH_DEFINES
 #include <cmath>
 #include <limits>
 #include <vector>
 
 #include "skia/ext/image_operations.h"
@@ skipping 377 matching lines @@
   SkAutoLockPixels lock_alpha(alpha);
   SkAutoLockPixels lock_masked(masked);
 
   for (int y = 0; y < rgb.height(); y++) {
     uint32* rgb_row = rgb.getAddr32(0, y);
     uint32* alpha_row = alpha.getAddr32(0, y);
     uint32* dst_row = masked.getAddr32(0, y);
 
     for (int x = 0; x < rgb.width(); x++) {
       uint32 alpha_pixel = alpha_row[x];
-      uint32 rgb_pixel = rgb_row[x];
+      SkColor rgb_pixel = SkUnPreMultiply::PMColorToColor(rgb_row[x]);
 
-      int alpha = SkColorGetA(alpha_pixel);
+      int alpha = SkAlphaMul(SkColorGetA(rgb_pixel), SkColorGetA(alpha_pixel));
       dst_row[x] = SkColorSetARGB(alpha,
                                   SkAlphaMul(SkColorGetR(rgb_pixel), alpha),
                                   SkAlphaMul(SkColorGetG(rgb_pixel), alpha),
                                   SkAlphaMul(SkColorGetB(rgb_pixel), alpha));
     }
   }
 
   return masked;
 }
 
@@ skipping 248 matching lines @@
   // Since bitmaps are refcounted, this copy will be fast.
   SkBitmap current = bitmap;
   while (current.width() >= min_w * 2 && current.height() >= min_h * 2 &&
          current.width() > 1 && current.height() > 1)
     current = DownsampleByTwo(current);
   return current;
 }
 
 }  // namespace skia