Fix memory leak in SkPictureRecord

Review URL: https://codereview.chromium.org/1197283007
diff --git a/src/core/SkPictureRecord.cpp b/src/core/SkPictureRecord.cpp
index 018da0b..32033b5 100644
--- a/src/core/SkPictureRecord.cpp
+++ b/src/core/SkPictureRecord.cpp
@@ -37,6 +37,7 @@
 }
 
 SkPictureRecord::~SkPictureRecord() {
+    fImageRefs.unrefAll();
     fPictureRefs.unrefAll();
     fTextBlobRefs.unrefAll();
 }