Add additive animation support for CSS property background-position

third_party/WebKit/Source/core/animation/PropertyInterpolationTypesMapping.cpp

Index: third_party/WebKit/Source/core/animation/PropertyInterpolationTypesMapping.cpp
diff --git a/third_party/WebKit/Source/core/animation/PropertyInterpolationTypesMapping.cpp b/third_party/WebKit/Source/core/animation/PropertyInterpolationTypesMapping.cpp
index 4cf12c7249e2452e05762b4835c67506aa291bb9..493cb64d12f9ae8875182fd8c98b08b800c90ac7 100644
--- a/third_party/WebKit/Source/core/animation/PropertyInterpolationTypesMapping.cpp
+++ b/third_party/WebKit/Source/core/animation/PropertyInterpolationTypesMapping.cpp
@@ -15,6 +15,7 @@
 #include "core/animation/CSSNumberInterpolationType.h"
 #include "core/animation/CSSPaintInterpolationType.h"
 #include "core/animation/CSSPathInterpolationType.h"
+#include "core/animation/CSSPositionAxisListInterpolationType.h"
 #include "core/animation/CSSShadowListInterpolationType.h"
 #include "core/animation/CSSValueInterpolationType.h"
 #include "core/animation/CSSVisibilityInterpolationType.h"
@@ -167,6 +168,12 @@ const InterpolationTypes* PropertyInterpolationTypesMapping::get(const PropertyH
         case CSSPropertyVisibility:
             applicableTypes->append(adoptPtr(new CSSVisibilityInterpolationType(cssProperty)));
             break;
+        case CSSPropertyBackgroundPositionX:
+        case CSSPropertyBackgroundPositionY:
+        case CSSPropertyWebkitMaskPositionX:
+        case CSSPropertyWebkitMaskPositionY:
+            applicableTypes->append(adoptPtr(new CSSPositionAxisListInterpolationType(cssProperty)));
+            break;
         default:
             // TODO(alancutter): Support all interpolable CSS properties here so we can stop falling back to the old StyleInterpolation implementation.
             if (CSSPropertyMetadata::isInterpolableProperty(cssProperty))