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/SkLerpXfermode.cpp b/src/effects/SkLerpXfermode.cpp
index c8389fe..0376a57 100644
--- a/src/effects/SkLerpXfermode.cpp
+++ b/src/effects/SkLerpXfermode.cpp
@@ -23,16 +23,20 @@
 
 SkLerpXfermode::SkLerpXfermode(unsigned scale256) : fScale256(scale256) {}
 
-SkLerpXfermode::SkLerpXfermode(SkReadBuffer& buffer)
-    : INHERITED(buffer) {
+#ifdef SK_SUPPORT_LEGACY_DEEPFLATTENING
+SkLerpXfermode::SkLerpXfermode(SkReadBuffer& buffer) : INHERITED(buffer) {
     fScale256 = buffer.readUInt();
 }
+#endif
 
 void SkLerpXfermode::flatten(SkWriteBuffer& buffer) const {
-    this->INHERITED::flatten(buffer);
     buffer.writeUInt(fScale256);
 }
 
+SkFlattenable* SkLerpXfermode::CreateProc(SkReadBuffer& buffer) {
+    return SkNEW_ARGS(SkLerpXfermode, (buffer.readUInt()));
+}
+
 void SkLerpXfermode::xfer32(SkPMColor dst[], const SkPMColor src[], int count,
                              const SkAlpha aa[]) const {
     const int scale = fScale256;