Make MAIL_PROVIDER_EXTRAS final

Change-Id: I330fddf66325df6870a0c671998268c5854b82fb

gerrit-server/src/main/java/com/google/gerrit/server/mail/receive/HtmlParser.java

diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/mail/receive/HtmlParser.java b/gerrit-server/src/main/java/com/google/gerrit/server/mail/receive/HtmlParser.java
index 8b07ded..14cb09a 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/mail/receive/HtmlParser.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/mail/receive/HtmlParser.java
@@ -29,7 +29,7 @@
 /** Provides functionality for parsing the HTML part of a {@link MailMessage}. */
 public class HtmlParser {
 
-  private static ImmutableList<String> MAIL_PROVIDER_EXTRAS =
+  private static final ImmutableList<String> MAIL_PROVIDER_EXTRAS =
       ImmutableList.of(
           "gmail_extra", // "On 01/01/2017 User<user@gmail.com> wrote:"
           "gmail_quote" // Used for quoting original content