SkImage doesn't use props, so don't need to store it

BUG=skia:
TBR=bsalomon

Review URL: https://codereview.chromium.org/1372153006
diff --git a/src/image/SkImage_Generator.cpp b/src/image/SkImage_Generator.cpp
index 1c1b263..5ce552c 100644
--- a/src/image/SkImage_Generator.cpp
+++ b/src/image/SkImage_Generator.cpp
@@ -17,7 +17,7 @@
 class SkImage_Generator : public SkImage_Base {
 public:
     SkImage_Generator(SkImageCacherator* cache)
-        : INHERITED(cache->info().width(), cache->info().height(), cache->uniqueID(), NULL)
+        : INHERITED(cache->info().width(), cache->info().height(), cache->uniqueID())
         , fCache(cache) // take ownership
     {}