Explicitly friend ::SkPrivateEffectInitializer to flattenables.

Looks like the -Wmicrosoft warnings in the bug are coming up specificically
because many the effects in SkLightingImageFilter are defined inside an
anonymous namespace (spanning, I think, lines 33-702 today).

BUG=skia:4091

No public API changes.
TBR=reed@google.com

Review URL: https://codereview.chromium.org/1311783004

include/core/SkFlattenable.h

@@ -13,6 +13,8 @@
 class SkReadBuffer;
 class SkWriteBuffer;
 
+class SkPrivateEffectInitializer;
+
 /*
  *  Flattening is straight-forward:
  *      1. call getFactory() so we have a function-ptr to recreate the subclass
@@ -47,7 +49,7 @@ class SkWriteBuffer;
 #define SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(flattenable)    \
     private:                                                                \
     static SkFlattenable* CreateProc(SkReadBuffer&);                        \
-    friend class SkPrivateEffectInitializer;                                \
+    friend class ::SkPrivateEffectInitializer;                              \
     public:                                                                 \
     Factory getFactory() const override { return CreateProc; }