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/SkXfermodeImageFilter.cpp b/src/effects/SkXfermodeImageFilter.cpp
index c876d5f..15ff92a 100644
--- a/src/effects/SkXfermodeImageFilter.cpp
+++ b/src/effects/SkXfermodeImageFilter.cpp
@@ -31,10 +31,18 @@
     SkSafeUnref(fMode);
 }
 
+#ifdef SK_SUPPORT_LEGACY_DEEPFLATTENING
 SkXfermodeImageFilter::SkXfermodeImageFilter(SkReadBuffer& buffer)
   : INHERITED(2, buffer) {
     fMode = buffer.readXfermode();
 }
+#endif
+
+SkFlattenable* SkXfermodeImageFilter::CreateProc(SkReadBuffer& buffer) {
+    SK_IMAGEFILTER_UNFLATTEN_COMMON(common, 2);
+    SkAutoTUnref<SkXfermode> mode(buffer.readXfermode());
+    return Create(mode, common.getInput(0), common.getInput(1), &common.cropRect());
+}
 
 void SkXfermodeImageFilter::flatten(SkWriteBuffer& buffer) const {
     this->INHERITED::flatten(buffer);