move SkChecksum and SkEmptyShader to private

TBR=reed@google.com

Author: reed@chromium.org

Review URL: https://codereview.chromium.org/366133002
diff --git a/src/effects/gradients/SkGradientShader.cpp b/src/effects/gradients/SkGradientShader.cpp
index 038044d..fe720b8 100644
--- a/src/effects/gradients/SkGradientShader.cpp
+++ b/src/effects/gradients/SkGradientShader.cpp
@@ -698,8 +698,6 @@
 ///////////////////////////////////////////////////////////////////////////////
 ///////////////////////////////////////////////////////////////////////////////
 
-#include "SkEmptyShader.h"
-
 // assumes colors is SkColor* and pos is SkScalar*
 #define EXPAND_1_COLOR(count)               \
     SkColor tmp[2];                         \
@@ -790,7 +788,7 @@
         return NULL;
     }
     if (start == end && startRadius == endRadius) {
-        return SkNEW(SkEmptyShader);
+        return SkShader::CreateEmptyShader();
     }
 
     EXPAND_1_COLOR(colorCount);