Disable external navigation for file downloads.

chrome/android/java/src/org/chromium/chrome/browser/externalnav/ExternalNavigationDelegate.java

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package org.chromium.chrome.browser.externalnav;
 
 import android.content.Intent;
 import android.content.pm.ResolveInfo;
 
 import org.chromium.chrome.browser.externalnav.ExternalNavigationHandler.OverrideUrlLoadingResult;
@@ skipping 89 matching lines @@
 
     /**
      * Check if Chrome is running in document mode.
      */
     boolean isDocumentMode();
 
     /**
      * @return Default SMS application's package name. Null if there isn't any.
      */
     String getDefaultSmsPackageName();
+
+    /**
+     * @return Whether the URL is a file download.
+     */
+    boolean isPdfDownload(String url);
 }