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/gm/gradient_matrix.cpp b/gm/gradient_matrix.cpp
index 275acc8..4f2915a 100644
--- a/gm/gradient_matrix.cpp
+++ b/gm/gradient_matrix.cpp
@@ -63,7 +63,7 @@
 
 static SkShader* make_linear_gradient(const SkPoint pts[2], const SkMatrix& localMatrix) {
     return SkGradientShader::CreateLinear(pts, gColors, NULL, SK_ARRAY_COUNT(gColors),
-                                          SkShader::kClamp_TileMode, 0, &localMatrix);
+                                          SkShader::kClamp_TileMode, NULL, 0, &localMatrix);
 }
 
 static SkShader* make_radial_gradient(const SkPoint pts[2], const SkMatrix& localMatrix) {
@@ -72,7 +72,7 @@
                SkScalarAve(pts[0].fY, pts[1].fY));
     float radius = (center - pts[0]).length();
     return SkGradientShader::CreateRadial(center, radius, gColors, NULL, SK_ARRAY_COUNT(gColors),
-                                          SkShader::kClamp_TileMode, 0, &localMatrix);
+                                          SkShader::kClamp_TileMode, NULL, 0, &localMatrix);
 }
 
 static void draw_gradients(SkCanvas* canvas,