Upstream: Initialize ChromeWebApkHost in WebappLaunchActivity.

chrome/android/webapk/libs/client/src/org/chromium/webapk/lib/client/WebApkValidator.java

 // Copyright 2016 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.webapk.lib.client;
 
 import static org.chromium.webapk.lib.common.WebApkConstants.WEBAPK_PACKAGE_PREFIX;
 
 import android.content.Context;
 import android.content.Intent;
@@ skipping 100 matching lines @@
         Log.d(TAG, "WebApk invalid");
         return false;
     }
 
     /**
      * Initializes the WebApkValidator with the expected signature that WebAPKs must be signed
      * with for the current host.
      * @param expectedSignature
      */
     public static void initWithBrowserHostSignature(byte[] expectedSignature) {
+        if (sExpectedSignature != null) {
+            return;
+        }
         sExpectedSignature = Arrays.copyOf(expectedSignature, expectedSignature.length);
     }
 }