Apply "type inference for generic instance creation" Java 7 feature

Since GWT 2.6.0 support for Java 7 is added. Simplify creation of
classes in GWT's client code.

Change-Id: I08ff2c189d2874a6b957072912912e4a6089cdd1

gerrit-gwtui/src/main/java/com/google/gerrit/client/Gerrit.java

@@ -571,7 +571,7 @@ public class Gerrit implements EntryPoint {
     menuLeft.clear();
     menuRight.clear();
 
-    menuBars = new HashMap<String, LinkMenuBar>();
+    menuBars = new HashMap<>();
 
     final boolean signedIn = isSignedIn();
     final GerritConfig cfg = getConfig();