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

src/core/SkDevice.cpp

Index: src/core/SkDevice.cpp
diff --git a/src/core/SkDevice.cpp b/src/core/SkDevice.cpp
index 6a09c0b9db8b2443a678d0e18176692f976a7382..e71c889451df139dd8e92ed28d9e4a2b57ebf084 100644
--- a/src/core/SkDevice.cpp
+++ b/src/core/SkDevice.cpp
@@ -134,10 +134,6 @@ void SkBaseDevice::EXPERIMENTAL_optimize(const SkPicture* picture) {
     // The base class doesn't perform any analysis but derived classes may
 }
 
-void SkBaseDevice::EXPERIMENTAL_purge(const SkPicture* picture) {
-    // Derived-classes may have data to purge but not the base class
-}
-
 bool SkBaseDevice::EXPERIMENTAL_drawPicture(SkCanvas* canvas, const SkPicture* picture) {
     // The base class doesn't perform any accelerated picture rendering
     return false;