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

src/core/SkCanvas.cpp

Index: src/core/SkCanvas.cpp
diff --git a/src/core/SkCanvas.cpp b/src/core/SkCanvas.cpp
index 4b1749a08b6876a30b4dc3db0e7129d9225828ce..504c9908abbb8015a65e7bfd7b77a2662d21154a 100644
--- a/src/core/SkCanvas.cpp
+++ b/src/core/SkCanvas.cpp
@@ -2386,13 +2386,6 @@ void SkCanvas::EXPERIMENTAL_optimize(const SkPicture* picture) {
     }
 }
 
-void SkCanvas::EXPERIMENTAL_purge(const SkPicture* picture) {
-    SkBaseDevice* device = this->getTopDevice();
-    if (NULL != device) {
-        device->EXPERIMENTAL_purge(picture);
-    }
-}
-
 void SkCanvas::drawPicture(const SkPicture* picture) {
     if (NULL != picture) {
         this->onDrawPicture(picture);