Ensure WebContentsObserver.destroy is called

content/public/android/java/src/org/chromium/content/browser/webcontents/WebContentsObserverProxy.java

Index: content/public/android/java/src/org/chromium/content/browser/webcontents/WebContentsObserverProxy.java
diff --git a/content/public/android/java/src/org/chromium/content/browser/webcontents/WebContentsObserverProxy.java b/content/public/android/java/src/org/chromium/content/browser/webcontents/WebContentsObserverProxy.java
index fdae8f3a3afe3a0c9de2649107d28757a127c843..730379a49f4534b3e1c0ebc1be782735694f1c3f 100644
--- a/content/public/android/java/src/org/chromium/content/browser/webcontents/WebContentsObserverProxy.java
+++ b/content/public/android/java/src/org/chromium/content/browser/webcontents/WebContentsObserverProxy.java
@@ -224,7 +224,14 @@ class WebContentsObserverProxy extends WebContentsObserver {
         // Java-based WebContents) are quite different, so we explicitly avoid
         // calling it here.
         ThreadUtils.assertOnUiThread();
+        for (mObserversIterator.rewind(); mObserversIterator.hasNext();) {
+            mObserversIterator.next().destroy();
+        }
+        // All observer destroy() implementations should result in their removal
+        // from the proxy.
+        assert mObservers.isEmpty();
         mObservers.clear();
+
         if (mNativeWebContentsObserverProxy != 0) {
             nativeDestroy(mNativeWebContentsObserverProxy);
             mNativeWebContentsObserverProxy = 0;