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/SkArithmeticMode.cpp b/src/effects/SkArithmeticMode.cpp
index 7d9997a..d3be826 100644
--- a/src/effects/SkArithmeticMode.cpp
+++ b/src/effects/SkArithmeticMode.cpp
@@ -47,6 +47,7 @@
         fEnforcePMColor = enforcePMColor;
     }
 
+#ifdef SK_SUPPORT_LEGACY_DEEPFLATTENING
     SkArithmeticMode_scalar(SkReadBuffer& buffer) : INHERITED(buffer) {
         fK[0] = buffer.readScalar();
         fK[1] = buffer.readScalar();
@@ -54,9 +55,9 @@
         fK[3] = buffer.readScalar();
         fEnforcePMColor = buffer.readBool();
     }
+#endif
 
     virtual void flatten(SkWriteBuffer& buffer) const SK_OVERRIDE {
-        INHERITED::flatten(buffer);
         buffer.writeScalar(fK[0]);
         buffer.writeScalar(fK[1]);
         buffer.writeScalar(fK[2]);
@@ -66,9 +67,20 @@
     SkScalar fK[4];
     bool fEnforcePMColor;
 
+    friend class SkArithmeticMode;
+
     typedef SkXfermode INHERITED;
 };
 
+SkFlattenable* SkArithmeticMode_scalar::CreateProc(SkReadBuffer& buffer) {
+    const SkScalar k1 = buffer.readScalar();
+    const SkScalar k2 = buffer.readScalar();
+    const SkScalar k3 = buffer.readScalar();
+    const SkScalar k4 = buffer.readScalar();
+    const bool enforcePMColor = buffer.readBool();
+    return Create(k1, k2, k3, k4, enforcePMColor);
+}
+
 static int pinToByte(int value) {
     if (value < 0) {
         value = 0;