Clean up other instances where Tab WindowAndroid was being set to null.

chrome/android/java/src/org/chromium/chrome/browser/tab/Tab.java

Index: chrome/android/java/src/org/chromium/chrome/browser/tab/Tab.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/tab/Tab.java b/chrome/android/java/src/org/chromium/chrome/browser/tab/Tab.java
index 221c50c7202d2ebb3556a088f60927255d5d3bfb..76d32aa1a716c9d8a4315c6c86522d9f17e584b0 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/tab/Tab.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/tab/Tab.java
@@ -1618,6 +1618,8 @@ public class Tab implements ViewGroup.OnHierarchyChangeListener,
      * @param windowAndroid The WindowAndroid to propagate.
      */
     public void updateWindowAndroid(WindowAndroid windowAndroid) {
+        // TODO(yusufo): mWindowAndroid can never be null until crbug.com/657007 is fixed.
+        assert windowAndroid != null;
         mWindowAndroid = windowAndroid;
         if (mContentViewCore != null) mContentViewCore.updateWindowAndroid(mWindowAndroid);
     }