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/SkLinearGradient.cpp b/src/effects/gradients/SkLinearGradient.cpp
index 9de4591..4f85da3 100644
--- a/src/effects/gradients/SkLinearGradient.cpp
+++ b/src/effects/gradients/SkLinearGradient.cpp
@@ -52,11 +52,11 @@
 
 ///////////////////////////////////////////////////////////////////////////////
 
-SkLinearGradient::SkLinearGradient(const SkPoint pts[2], const Descriptor& desc,
-                                   const SkMatrix* localMatrix)
-    : SkGradientShaderBase(desc, localMatrix)
+SkLinearGradient::SkLinearGradient(const SkPoint pts[2], const Descriptor& desc)
+    : SkGradientShaderBase(desc)
     , fStart(pts[0])
-    , fEnd(pts[1]) {
+    , fEnd(pts[1])
+{
     pts_to_unit_matrix(pts, &fPtsToUnit);
 }