[Android] Add proxy for Java-based WebContentsObservers

base/android/java/src/org/chromium/base/WeakObserverList.java

Index: base/android/java/src/org/chromium/base/WeakObserverList.java
diff --git a/base/android/java/src/org/chromium/base/WeakObserverList.java b/base/android/java/src/org/chromium/base/WeakObserverList.java
new file mode 100644
index 0000000000000000000000000000000000000000..ae1f31bfc3a74e82c76c7ede2a950e3c8a6263b8
--- /dev/null
+++ b/base/android/java/src/org/chromium/base/WeakObserverList.java
@@ -0,0 +1,27 @@
+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package org.chromium.base;
+
+import java.lang.ref.WeakReference;
+
+/**
+ * A container for a list of weakly referenced observers. See BaseObserverList for details.
+ *
+ * @param <E> The type of observers that this list should hold.
+ */
+public final class WeakObserverList<E> extends BaseObserverList<E, WeakReference<E>> {
+
+    public WeakObserverList() {}
+
+    @Override
+    final WeakReference<E> wrap(E obs) {
+        return new WeakReference<E>(obs);
+    }
+
+    @Override
+    final E unwrap(WeakReference<E> wrapper) {
+        return wrapper != null ? wrapper.get() : null;
+    }
+}