SideBySide2: Clarify diffChunkNav direction

Instead of abusing a boolean true as previous and false as next,
define an enum with symbols PREV and NEXT, making it more clear
which direction navigation will go in the file.

Change-Id: I61f2279aa1239ce88727235d88e125fc0cec1ee9

gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/Direction.java

@@ -0,0 +1,20 @@
+// Copyright (C) 2013 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.client.diff;
+
+/** Direction of traversal in an ordered list. */
+public enum Direction {
+  PREV, NEXT
+}