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

Reason for revert:
does not address all legacy callsites in chrome.

e.g.

[13:45:32.091872] ../../ui/native_theme/native_theme_base.cc:608:76: error: no matching function for call to ‘SkGradientShader::CreateLinear(SkPoint [3], SkColor [3], NULL, int, SkShader::TileMode, NULL)’
[13:45:32.091919]            gradient_bounds, colors, NULL, 3, SkShader::kClamp_TileMode, NULL));


Original issue's description:
> remove unused (by clients) SkUnitMapper
>
> BUG=skia:
>
> Committed: http://code.google.com/p/skia/source/detail?r=14761

R=robertphillips@google.com, scroggo@google.com, george@mozilla.com
TBR=george@mozilla.com, robertphillips@google.com, scroggo@google.com
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Author: reed@google.com

Review URL: https://codereview.chromium.org/287063009

git-svn-id: http://skia.googlecode.com/svn/trunk@14763 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/effects/gradients/SkGradientShaderPriv.h b/src/effects/gradients/SkGradientShaderPriv.h
index 9613537..a699c4c 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;