Extract most of the mutable state of SkShader into a separate Context object.

src/effects/gradients/SkGradientShader.cpp

Index: src/effects/gradients/SkGradientShader.cpp
diff --git a/src/effects/gradients/SkGradientShader.cpp b/src/effects/gradients/SkGradientShader.cpp
index 5b0b9c763da184ec9b732112cbb2d76ed02ece26..8a45606f3145916c32d91792d6352ae298e8577e 100644
--- a/src/effects/gradients/SkGradientShader.cpp
+++ b/src/effects/gradients/SkGradientShader.cpp
@@ -224,6 +224,7 @@ bool SkGradientShaderBase::isOpaque() const {
     return fColorsAreOpaque;
 }
 
+#if 0
 bool SkGradientShaderBase::setContext(const SkBitmap& device,
                                  const SkPaint& paint,
                                  const SkMatrix& matrix) {
@@ -258,6 +259,7 @@ bool SkGradientShaderBase::setContext(const SkBitmap& device,
     this->setCacheAlpha(paintAlpha);
     return true;
 }
+#endif
 
 void SkGradientShaderBase::setCacheAlpha(U8CPU alpha) const {
     // if the new alpha differs from the previous time we were called, inval our cache