Revert of change all factories to return their base-class

src/effects/SkDisplacementMapEffect.cpp

Index: src/effects/SkDisplacementMapEffect.cpp
diff --git a/src/effects/SkDisplacementMapEffect.cpp b/src/effects/SkDisplacementMapEffect.cpp
index 2b35036bc55ae7e2c33412218868ff4bbaef738b..a3fff39b46f062dc1f0a93293f52ab8991305f91 100644
--- a/src/effects/SkDisplacementMapEffect.cpp
+++ b/src/effects/SkDisplacementMapEffect.cpp
@@ -164,12 +164,12 @@
 
 ///////////////////////////////////////////////////////////////////////////////
 
-SkImageFilter* SkDisplacementMapEffect::Create(ChannelSelectorType xChannelSelector,
-                                               ChannelSelectorType yChannelSelector,
-                                               SkScalar scale,
-                                               SkImageFilter* displacement,
-                                               SkImageFilter* color,
-                                               const CropRect* cropRect) {
+SkDisplacementMapEffect* SkDisplacementMapEffect::Create(ChannelSelectorType xChannelSelector,
+                                                         ChannelSelectorType yChannelSelector,
+                                                         SkScalar scale,
+                                                         SkImageFilter* displacement,
+                                                         SkImageFilter* color,
+                                                         const CropRect* cropRect) {
     if (!channel_selector_type_is_valid(xChannelSelector) ||
         !channel_selector_type_is_valid(yChannelSelector)) {
         return nullptr;