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

src/core/SkPicture.cpp

Index: src/core/SkPicture.cpp
diff --git a/src/core/SkPicture.cpp b/src/core/SkPicture.cpp
index d57de97d78c5f67a3c3da1c9a0087b9497e7a052..0b7a1dbcc592702ec3cf9185f3bfcded0b32bf79 100644
--- a/src/core/SkPicture.cpp
+++ b/src/core/SkPicture.cpp
@@ -144,7 +144,9 @@ static SkPicture* backport(const SkRecord& src, int width, int height) {
 }
 
 // fRecord OK
-SkPicture::~SkPicture() {}
+SkPicture::~SkPicture() {
+    this->callDeletionListeners();
+}
 
 #ifdef SK_SUPPORT_LEGACY_PICTURE_CLONE
 // fRecord TODO, fix by deleting this method
@@ -492,3 +494,21 @@ SkPicture::SkPicture(int width, int height, SkRecord* record)
     , fRecordWillPlayBackBitmaps(SkRecordWillPlaybackBitmaps(*record)) {
     this->needsNewGenID();
 }
+
+// Note that we are assuming that this entry point will only be called from
+// one thread. Currently the only client of this method is 
+// SkGpuDevice::EXPERIMENTAL_optimize which should be only called from a single
+// thread.
+void SkPicture::addDeletionListener(DeletionListener* listener) const {
+    SkASSERT(NULL != listener);
+
+    *fDeletionListeners.append() = SkRef(listener);
+}
+
+void SkPicture::callDeletionListeners() {
+    for (int i = 0; i < fDeletionListeners.count(); ++i) {
+        fDeletionListeners[i]->onDeletion(this->uniqueID());
+    }
+
+    fDeletionListeners.unrefAll();
+}