Move clip off of draw target

BUG=skia:

Review URL: https://codereview.chromium.org/947443003
diff --git a/src/gpu/GrSWMaskHelper.h b/src/gpu/GrSWMaskHelper.h
index c180971..a69e4be 100644
--- a/src/gpu/GrSWMaskHelper.h
+++ b/src/gpu/GrSWMaskHelper.h
@@ -18,6 +18,7 @@
 #include "SkTextureCompressor.h"
 #include "SkTypes.h"
 
+class GrClip;
 class GrContext;
 class GrTexture;
 class SkPath;