android: Switch CleanupReference to PhantomReference

content/public/android/java/src/org/chromium/content/common/CleanupReference.java

Index: content/public/android/java/src/org/chromium/content/common/CleanupReference.java
diff --git a/content/public/android/java/src/org/chromium/content/common/CleanupReference.java b/content/public/android/java/src/org/chromium/content/common/CleanupReference.java
index 95e61cea2fce437ca57e71da3dc6a13c7c6227c8..6c6fcd7195a2e3f0aa188a879bd0bc213c04dc66 100644
--- a/content/public/android/java/src/org/chromium/content/common/CleanupReference.java
+++ b/content/public/android/java/src/org/chromium/content/common/CleanupReference.java
@@ -12,8 +12,8 @@ import org.chromium.base.Log;
 import org.chromium.base.ThreadUtils;
 import org.chromium.base.TraceEvent;
 
+import java.lang.ref.PhantomReference;
 import java.lang.ref.ReferenceQueue;
-import java.lang.ref.WeakReference;
 import java.util.HashSet;
 import java.util.Set;
 
@@ -25,12 +25,13 @@ import java.util.Set;
  * cleaned up in response to java side GC of API objects. (Private/internal
  * interfaces should always favor explicit resource releases / destroy()
  * protocol for this rather than depend on GC to trigger native cleanup).
- * NOTE this uses WeakReference rather than PhantomReference, to avoid delaying the
- * cleanup processing until after finalizers (if any) have run. In general usage of
- * this class indicates the client does NOT use finalizers anyway (Good), so this should
- * not be a visible difference in practice.
+ *
+ * NOTE Using PhantonReference instead of WeakReference is required for correctness.
+ * WeakReferences are enqueued before finalizers are called, and finalizers can
+ * resurrect the referent object. PhantomReference does delay clean up more compared
+ * to WeakReference.
  */
-public class CleanupReference extends WeakReference<Object> {
+public class CleanupReference extends PhantomReference<Object> {
     private static final String TAG = "cr.CleanupReference";
 
     private static final boolean DEBUG = false;  // Always check in as false!