Simplify flattening to just write enough to call the factory/public-constructor for the class. We want to *not* rely on private constructors, and not rely on calling through the inheritance hierarchy for either flattening or unflattening(CreateProc).

Refactoring pattern:

1. guard the existing constructor(readbuffer) with the legacy build-flag
2. If you are a instancable subclass, implement CreateProc(readbuffer) to create a new instances from the buffer params (or return NULL).

If you're a shader subclass
1. You must read/write the local matrix if your class accepts that in its factory/constructor, else ignore it.

R=robertphillips@google.com, mtklein@google.com, senorblanco@google.com, senorblanco@chromium.org, sugoi@chromium.org

Author: reed@google.com

Review URL: https://codereview.chromium.org/395603002
diff --git a/src/effects/SkLayerRasterizer.cpp b/src/effects/SkLayerRasterizer.cpp
index 9b7bdca..b331a03 100644
--- a/src/effects/SkLayerRasterizer.cpp
+++ b/src/effects/SkLayerRasterizer.cpp
@@ -148,12 +148,18 @@
     return true;
 }
 
+#ifdef SK_SUPPORT_LEGACY_DEEPFLATTENING
 SkLayerRasterizer::SkLayerRasterizer(SkReadBuffer& buffer)
     : SkRasterizer(buffer), fLayers(ReadLayers(buffer)) {}
+#endif
+
+SkFlattenable* SkLayerRasterizer::CreateProc(SkReadBuffer& buffer) {
+    return SkNEW_ARGS(SkLayerRasterizer, (ReadLayers(buffer)));
+}
 
 SkDeque* SkLayerRasterizer::ReadLayers(SkReadBuffer& buffer) {
     int count = buffer.readInt();
-
+    
     SkDeque* layers = SkNEW_ARGS(SkDeque, (sizeof(SkLayerRasterizer_Rec)));
     for (int i = 0; i < count; i++) {
         SkLayerRasterizer_Rec* rec = (SkLayerRasterizer_Rec*)layers->push_back();