[proguard] Add VisibleToTesting to clearHistory on NavigationController.

Only used in tests currently, so we need to prevent it from being removed.

BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/591283002

Cr-Commit-Position: refs/heads/master@{#296103}

content/public/android/java/src/org/chromium/content_public/browser/NavigationController.java

@@ -4,6 +4,8 @@
 
 package org.chromium.content_public.browser;
 
+import org.chromium.base.VisibleForTesting;
+
 /**
  * The NavigationController Java wrapper to allow communicating with the native
  * NavigationController object.
@@ -90,6 +92,7 @@ public interface NavigationController {
      * Clears NavigationController's page history in both backwards and
      * forwards directions.
      */
+    @VisibleForTesting
     public void clearHistory();
 
     /**