Bug 848491 - Re-apply bug 795538 - Ensure we use the correct colour (and alpha) for the clamp values r=mattwoodrow

gfx/skia/src/effects/gradients/SkGradientShader.cpp

@@ -500,12 +500,14 @@ const SkPMColor* SkGradientShaderBase::getCache32() const {
     }
 
     // Write the clamp colours into the first and last entries of fCache32
-    fCache32[kCache32ClampLower] = SkPackARGB32(fCacheAlpha,
+    fCache32[kCache32ClampLower] = SkPremultiplyARGBInline(SkMulDiv255Round(SkColorGetA(fOrigColors[0]),
+                                                                            fCacheAlpha),
                                                            SkColorGetR(fOrigColors[0]),
                                                            SkColorGetG(fOrigColors[0]),
                                                            SkColorGetB(fOrigColors[0]));
 
-    fCache32[kCache32ClampUpper] = SkPackARGB32(fCacheAlpha,
+    fCache32[kCache32ClampUpper] = SkPremultiplyARGBInline(SkMulDiv255Round(SkColorGetA(fOrigColors[fColorCount - 1]),
+                                                                            fCacheAlpha),
                                                            SkColorGetR(fOrigColors[fColorCount - 1]),
                                                            SkColorGetG(fOrigColors[fColorCount - 1]),
                                                            SkColorGetB(fOrigColors[fColorCount - 1]));