Disable external navigation for file downloads.

chrome/android/javatests/src/org/chromium/chrome/browser/externalnav/ExternalNavigationHandlerTest.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.annotation.SuppressLint;
 import android.content.Context;
 import android.content.Intent;
 import android.content.pm.ActivityInfo;
@@ skipping 838 matching lines @@
         @Override
         public boolean isDocumentMode() {
             return FeatureUtilities.isDocumentMode(mContext);
         }
 
         @Override
         public String getDefaultSmsPackageName() {
             return defaultSmsPackageName;
         }
 
+        @Override
+        public boolean isPdfDownload(String url) {
+            return false;
+        }
+
         public void reset() {
             startActivityIntent = null;
             startIncognitoIntentCalled = false;
             startFileIntentCalled = false;
         }
 
         public void setCanResolveActivity(boolean value) {
             mQueryIntentOverride = value;
         }
 
@@ skipping 151 matching lines @@
             return new TestPackageManager();
         }
 
         @Override
         public String getPackageName() {
             return "test.app.name";
         }
 
     }
 }