Renderer: Use native Function from Java 8 instead of Guava

And convert callers to use Stream API rather than FluentIterable.

Change-Id: I4fe140b0ddd240b4c328c2393d975ec6203b0007

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

diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/Renderer.java b/gitiles-servlet/src/main/java/com/google/gitiles/Renderer.java
index 86a4a9c..d4c2872 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/Renderer.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/Renderer.java
@@ -19,7 +19,6 @@
 import static com.google.common.base.Preconditions.checkState;
 import static java.nio.charset.StandardCharsets.UTF_8;
 
-import com.google.common.base.Function;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.MapMaker;
@@ -39,6 +38,7 @@
 import java.net.URL;
 import java.util.Map;
 import java.util.concurrent.ConcurrentMap;
+import java.util.function.Function;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;