mojo: Fix memory leak in java router

R=ppi@chromium.org

Review URL: https://codereview.chromium.org/600003002

Cr-Commit-Position: refs/heads/master@{#296436}

mojo/public/java/bindings/src/org/chromium/mojo/bindings/RouterImpl.java

@@ -176,6 +176,7 @@ public class RouterImpl implements Router {
             if (responder == null) {
                 return false;
             }
+            mResponders.remove(requestId);
             return responder.accept(message);
         } else {
             if (mIncomingMessageReceiver != null) {