add localMatrix to gradient Descriptor

this consolidation will also help transisition to new flattening pattern, where we want to
have a flatten/unflatten method on all of the common gradient params (i.e. Descriptor).

BUG=skia:
R=egdaniel@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/461643002
diff --git a/src/effects/gradients/SkRadialGradient.h b/src/effects/gradients/SkRadialGradient.h
index ac79a57..b7dbcbd 100644
--- a/src/effects/gradients/SkRadialGradient.h
+++ b/src/effects/gradients/SkRadialGradient.h
@@ -13,8 +13,7 @@
 
 class SkRadialGradient : public SkGradientShaderBase {
 public:
-    SkRadialGradient(const SkPoint& center, SkScalar radius, const Descriptor&,
-                     const SkMatrix* localMatrix);
+    SkRadialGradient(const SkPoint& center, SkScalar radius, const Descriptor&);
 
     virtual size_t contextSize() const SK_OVERRIDE;