Add SkPurgeGlobalDiscardableMemoryPool, needed to replace SkImageRef

src/lazy/SkDiscardableMemoryPool.cpp

Index: src/lazy/SkDiscardableMemoryPool.cpp
diff --git a/src/lazy/SkDiscardableMemoryPool.cpp b/src/lazy/SkDiscardableMemoryPool.cpp
index 961216963e39951fdce5b866b4617d57340999a9..4fb8ae4fe1fbc626453bf6eecd009dac5183b9fa 100644
--- a/src/lazy/SkDiscardableMemoryPool.cpp
+++ b/src/lazy/SkDiscardableMemoryPool.cpp
@@ -271,4 +271,8 @@ SkDiscardableMemoryPool* SkGetGlobalDiscardableMemoryPool() {
     return gPool;
 }
 
+// defined in SkImageGenerator.h
+void SkPurgeGlobalDiscardableMemoryPool() {
+    SkGetGlobalDiscardableMemoryPool()->dumpPool();
+}
 ////////////////////////////////////////////////////////////////////////////////