Add auto purging for SkPicture-related Ganesh resources (esp. layers)

include/core/SkCanvas.h

Index: include/core/SkCanvas.h
diff --git a/include/core/SkCanvas.h b/include/core/SkCanvas.h
index 6bd3cc5363175234c8fcd9665f2390c95f84033c..d68a9c50ab80d4a6fdd640756f9f1ce96eca620a 100644
--- a/include/core/SkCanvas.h
+++ b/include/core/SkCanvas.h
@@ -964,12 +964,6 @@ public:
     */
     void EXPERIMENTAL_optimize(const SkPicture* picture);
 
-    /** PRIVATE / EXPERIMENTAL -- do not call
-        Purge all the discardable optimization information associated with
-        'picture'. If NULL is passed in, purge all discardable information.
-    */
-    void EXPERIMENTAL_purge(const SkPicture* picture);
-
     /** Draw the picture into this canvas. This method effective brackets the
         playback of the picture's draw calls with save/restore, so the state
         of this canvas will be unchanged after this call.