Revert of Remove unused (by clients) SkUnitMapper (https://codereview.chromium.org/283273002/)

src/animator/SkDrawGradient.h

Index: src/animator/SkDrawGradient.h
diff --git a/src/animator/SkDrawGradient.h b/src/animator/SkDrawGradient.h
index fa46a9e629882f4507ff245fc5042aaa1d04bad7..ff79e3fb912158f31b6cc3bb77514597413bebc0 100644
--- a/src/animator/SkDrawGradient.h
+++ b/src/animator/SkDrawGradient.h
@@ -14,6 +14,8 @@
 #include "SkDrawShader.h"
 #include "SkIntArray.h"
 
+class SkUnitMapper;
+
 class SkDrawGradient : public SkDrawShader {
     DECLARE_PRIVATE_MEMBER_INFO(DrawGradient);
     SkDrawGradient();
@@ -28,6 +30,7 @@
     SkString unitMapper;
     SkTDColorArray fColors;
     SkTDDrawColorArray fDrawColors;
+    SkUnitMapper* fUnitMapper;
     int addPrelude();
 private:
     typedef SkDrawShader INHERITED;