Update unit tests after SkFlattenable::getFactory const-ification.

cc/test/skia_common.cc

Index: cc/test/skia_common.cc
diff --git a/cc/test/skia_common.cc b/cc/test/skia_common.cc
index 67c72f47c3bce4ec58ebadf6f849715120bee198..49c984dcc25685262151e25f75409ba784023fba 100644
--- a/cc/test/skia_common.cc
+++ b/cc/test/skia_common.cc
@@ -17,7 +17,7 @@ TestPixelRef::TestPixelRef(int width, int height)
 
 TestPixelRef::~TestPixelRef() {}
 
-SkFlattenable::Factory TestPixelRef::getFactory() { return NULL; }
+SkFlattenable::Factory TestPixelRef::getFactory() const { return NULL; }
 
 void* TestPixelRef::onLockPixels(SkColorTable** color_table) {
   return pixels_.get();
@@ -36,7 +36,7 @@ TestLazyPixelRef::TestLazyPixelRef(int width, int height)
 
 TestLazyPixelRef::~TestLazyPixelRef() {}
 
-SkFlattenable::Factory TestLazyPixelRef::getFactory() { return NULL; }
+SkFlattenable::Factory TestLazyPixelRef::getFactory() const { return NULL; }
 
 void* TestLazyPixelRef::onLockPixels(SkColorTable** color_table) {
   return pixels_.get();