make set3DMask virtual, so we can safely notify the shadercontext

include/core/SkShader.h

Index: include/core/SkShader.h
diff --git a/include/core/SkShader.h b/include/core/SkShader.h
index 8530023671a442cbbd57c82e85d5865afc3d2e43..6effe797f750d767f985ec331adaa8d3ba09cab5 100644
--- a/include/core/SkShader.h
+++ b/include/core/SkShader.h
@@ -185,6 +185,9 @@ public:
             return SkShader::CanCallShadeSpan16(this->getFlags());
         }
 
+        // Notification from blitter::blitMask in case we need to see the non-alpha channels
+        virtual void set3DMask(const SkMask*) {}
+
     protected:
         // Reference to shader, so we don't have to dupe information.
         const SkShader& fShader;