Renderer: Use lambda instead of named Function subclass

Change-Id: I74336bdc9cdec3bc3716901fecafc9676f5e8907

gitiles-servlet/src/main/java/com/google/gitiles/DebugRenderer.java

diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/DebugRenderer.java b/gitiles-servlet/src/main/java/com/google/gitiles/DebugRenderer.java
index 5a54cea..5ec22f2 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/DebugRenderer.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/DebugRenderer.java
@@ -33,10 +33,10 @@
       final String soyTemplatesRoot,
       String siteTitle) {
     super(
-        new FileUrlMapper(soyTemplatesRoot + File.separator),
+        fileUrlMapper(soyTemplatesRoot + File.separator),
         ImmutableMap.<String, String>of(),
         staticPrefix,
-        FluentIterable.from(customTemplatesFilenames).transform(new FileUrlMapper()),
+        FluentIterable.from(customTemplatesFilenames).transform(fileUrlMapper()),
         siteTitle);
   }