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

src/effects/gradients/SkGradientShaderPriv.h

Index: src/effects/gradients/SkGradientShaderPriv.h
diff --git a/src/effects/gradients/SkGradientShaderPriv.h b/src/effects/gradients/SkGradientShaderPriv.h
index 96135378032d52ade5e27b2c573408206543eb37..a699c4ce00b7e4e1672d45517592db1395e8d90f 100644
--- a/src/effects/gradients/SkGradientShaderPriv.h
+++ b/src/effects/gradients/SkGradientShaderPriv.h
@@ -14,6 +14,7 @@
 #include "SkReadBuffer.h"
 #include "SkWriteBuffer.h"
 #include "SkMallocPixelRef.h"
+#include "SkUnitMapper.h"
 #include "SkUtils.h"
 #include "SkTemplates.h"
 #include "SkBitmapCache.h"
@@ -93,6 +94,7 @@
         const SkScalar*     fPos;
         int                 fCount;
         SkShader::TileMode  fTileMode;
+        SkUnitMapper*       fMapper;
         uint32_t            fGradFlags;
     };
 
@@ -199,6 +201,7 @@
     virtual void flatten(SkWriteBuffer&) const SK_OVERRIDE;
     SK_TO_STRING_OVERRIDE()
 
+    SkUnitMapper* fMapper;
     SkMatrix    fPtsToUnit;     // set by subclass
     TileMode    fTileMode;
     TileProc    fTileProc;