Factory methods for heap-allocated SkPathEffect and SkXfermode objects.

This is part of an effort to ensure that all SkPaint effects can only be
allocated on the heap.

This patch makes the constructors of SkPathEffect, SkXfermode and
their subclasses non-public and instead provides factory methods for
creating these objects on the heap. We temporarily keep the constructors
of the following classes public to not break Chrome/Blink:

SkXfermode
SkCornerPathEffect
SkDashPathEffect

BUG=skia:2187
R=scroggo@google.com, reed@google.com, mtklein@google.com, bungeman@google.com

Author: dominikg@chromium.org

Review URL: https://codereview.chromium.org/166583002

git-svn-id: http://skia.googlecode.com/svn/trunk@13519 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/include/effects/SkPixelXorXfermode.h b/include/effects/SkPixelXorXfermode.h
index 9472d94..cfd59ec 100644
--- a/include/effects/SkPixelXorXfermode.h
+++ b/include/effects/SkPixelXorXfermode.h
@@ -17,7 +17,9 @@
 */
 class SK_API SkPixelXorXfermode : public SkXfermode {
 public:
-    SkPixelXorXfermode(SkColor opColor) : fOpColor(opColor) {}
+    static SkPixelXorXfermode* Create(SkColor opColor) {
+        return SkNEW_ARGS(SkPixelXorXfermode, (opColor));
+    }
 
     SK_DEVELOPER_TO_STRING()
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkPixelXorXfermode)
@@ -29,6 +31,11 @@
     // override from SkXfermode
     virtual SkPMColor xferColor(SkPMColor src, SkPMColor dst) const;
 
+#ifdef SK_SUPPORT_LEGACY_PUBLICEFFECTCONSTRUCTORS
+public:
+#endif
+    SkPixelXorXfermode(SkColor opColor) : fOpColor(opColor) {}
+
 private:
     SkColor fOpColor;