Ensure constants are declared as immutable collections

This communicates important semantic guarantees to callers, as described
in the ImmutableCollection javadoc. It also makes calls to mutating
methods result in compiler warnings.

PiperOrigin-RevId: 148916986
Change-Id: I5a400daca8b7a339e9cd7298e1ed1f6db3c54bfc

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 73718fb..86a4a9c 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/Renderer.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/Renderer.java
@@ -37,7 +37,6 @@
 import java.io.OutputStream;
 import java.net.MalformedURLException;
 import java.net.URL;
-import java.util.List;
 import java.util.Map;
 import java.util.concurrent.ConcurrentMap;
 import javax.servlet.http.HttpServletRequest;
@@ -53,7 +52,7 @@
   // Must match .streamingPlaceholder.
   private static final String PLACEHOLDER = "id=\"STREAMED_OUTPUT_BLOCK\"";
 
-  private static final List<String> SOY_FILENAMES =
+  private static final ImmutableList<String> SOY_FILENAMES =
       ImmutableList.of(
           "BlameDetail.soy",
           "Common.soy",
@@ -67,7 +66,7 @@
           "RevisionDetail.soy",
           "RepositoryIndex.soy");
 
-  public static final Map<String, String> STATIC_URL_GLOBALS =
+  public static final ImmutableMap<String, String> STATIC_URL_GLOBALS =
       ImmutableMap.of(
           "gitiles.BASE_CSS_URL", "base.css",
           "gitiles.DOC_CSS_URL", "doc.css",