In SkPixmap.cpp, change SkAlphaMul to SkMulDiv255.

tests/BitmapTest.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBitmap.h"
 #include "SkMallocPixelRef.h"
 #include "Test.h"
@@ skipping 127 matching lines @@
         SkBitmap copy;
         if (!source.copyTo(&copy, ct)) {
             ERRORF(r, "SkBitmap::copy failed %d", (int)ct);
             continue;
         }
         SkAutoLockPixels autoLockPixels1(copy);
         SkAutoLockPixels autoLockPixels2(source);
         REPORTER_ASSERT(r, source.getColor(0, 0) == copy.getColor(0, 0));
     }
 }
+
+static void test_erasecolor_premul(skiatest::Reporter* reporter, SkColorType ct, SkColor input,
+                                   SkColor expected) {
+  SkBitmap bm;
+  bm.allocPixels(SkImageInfo::Make(1, 1, ct, kPremul_SkAlphaType));
+  bm.eraseColor(input);
+  SkDebugf("expected: %x actual: %x\n", expected, bm.getColor(0, 0));
+  REPORTER_ASSERT(reporter, bm.getColor(0, 0) == expected);
+}
+
+/**
+ *  This test checks that eraseColor premultiplies the color correctly.
+ */
+DEF_TEST(Bitmap_eraseColor_Premul, r) {
+    SkColor color = 0x80FF0080;
+    test_erasecolor_premul(r, kAlpha_8_SkColorType, color, 0x80000000);
+    test_erasecolor_premul(r, kRGB_565_SkColorType, color, 0xFF840042);
+    test_erasecolor_premul(r, kARGB_4444_SkColorType, color, 0x88FF0080);
+    test_erasecolor_premul(r, kRGBA_8888_SkColorType, color, color);
+    test_erasecolor_premul(r, kBGRA_8888_SkColorType, color, color);
+}