Fix missing synchronized modifier

Change-Id: Ied70a86bedb08adcf8353b189367172459f55292

java/src/com/android/inputmethod/latin/UserHistoryDictionary.java

@@ -162,7 +162,7 @@ public class UserHistoryDictionary extends ExpandableDictionary {
      * Return whether the passed charsequence is in the dictionary.
      */
     @Override
-    public boolean isValidWord(final CharSequence word) {
+    public synchronized boolean isValidWord(final CharSequence word) {
         // TODO: figure out what is the correct thing to do here.
         return false;
     }