Fall back to legacy add to homescreen behavior on non-GMS device or GMS isn't availabe.

chrome/browser/android/webapk/webapk_installer.h

 // 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.
 
 #ifndef CHROME_BROWSER_ANDROID_WEBAPK_WEBAPK_INSTALLER_H_
 #define CHROME_BROWSER_ANDROID_WEBAPK_WEBAPK_INSTALLER_H_
 
 #include <jni.h>
 #include <map>
 #include <memory>
@@ skipping 106 matching lines @@
       const base::android::ScopedJavaLocalRef<jstring>& java_file_path,
       const base::android::ScopedJavaLocalRef<jstring>& java_package_name);
 
   // Starts update using the downloaded WebAPK. Returns whether the updating
   // could be started. The updating may still fail if true is returned.
   // |file_path| is the file path that the WebAPK was downloaded to.
   virtual bool StartUpdateUsingDownloadedWebApk(
       JNIEnv* env,
       const base::android::ScopedJavaLocalRef<jstring>& java_file_path);
 
-  // Returns whether the Google Play install delegate is available.
+  // Returns whether Google Play Service can be used and the install delegate is

[dominickn, 2017/01/18 00:07:02]

nit: "Google Play Services"

[Xi Han, 2017/01/18 14:53:27]

Done.

+  // available.
   // Note: it is possible that this delegate is null even when installing
   // WebAPKs using Google Play is enabled.
-  virtual bool HasGooglePlayWebApkInstallDelegate();
+  virtual bool CanUseGooglePlayInstallService();
 
   // Called when the package name of the WebAPK is available and the install
   // or update request is handled by Google Play.
   virtual bool InstallOrUpdateWebApkFromGooglePlay(
       const std::string& package_name,
       int version,
       const std::string& token);
 
   // Called when the request to install the WebAPK is sent to Google Play.
   void OnSuccess();
@@ skipping 137 matching lines @@
   // Points to the Java Object.
   base::android::ScopedJavaGlobalRef<jobject> java_ref_;
 
   // Used to get |weak_ptr_|.
   base::WeakPtrFactory<WebApkInstaller> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(WebApkInstaller);
 };
 
 #endif  // CHROME_BROWSER_ANDROID_WEBAPK_WEBAPK_INSTALLER_H_