Oilpan: fix build after r182644.

Source/modules/encryptedmedia/HTMLMediaElementEncryptedMedia.cpp

Index: Source/modules/encryptedmedia/HTMLMediaElementEncryptedMedia.cpp
diff --git a/Source/modules/encryptedmedia/HTMLMediaElementEncryptedMedia.cpp b/Source/modules/encryptedmedia/HTMLMediaElementEncryptedMedia.cpp
index e3e2d6eea141e5501ca34c85eeddd2ae09ef89f2..46ffd93f10861149d01eddbff0f021bd0be8a2ae 100644
--- a/Source/modules/encryptedmedia/HTMLMediaElementEncryptedMedia.cpp
+++ b/Source/modules/encryptedmedia/HTMLMediaElementEncryptedMedia.cpp
@@ -63,7 +63,8 @@ private:
 
     void reportSetFailed(ExceptionCode, const String& errorMessage);
 
-    RawPtrWillBeMember<HTMLMediaElement> m_element;
+    // Keep media element alive until promise is fulfilled
+    RefPtrWillBePersistent<HTMLMediaElement> m_element;
     Persistent<MediaKeys> m_newMediaKeys;
     Timer<SetMediaKeysHandler> m_timer;
 };