Fix/suppress new warnings introduced in Chromium roll.

TBR=henrika@webrtc.org

Bug: webrtc:6597
Change-Id: Id26945a7be05250673b58de8220f78bc62886688
Reviewed-on: https://webrtc-review.googlesource.com/16860
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20477}

sdk/android/api/org/webrtc/MediaConstraints.java

@@ -36,6 +36,7 @@ public class MediaConstraints {
       return value;
     }
 
+    @Override
     public String toString() {
       return key + ": " + value;
     }
@@ -77,6 +78,7 @@ public class MediaConstraints {
     return builder.append("]").toString();
   }
 
+  @Override
   public String toString() {
     return "mandatory: " + stringifyKeyValuePairList(mandatory) + ", optional: "
         + stringifyKeyValuePairList(optional);