[Android] Add AndroidSpellChecker feature to allow for gradual rollout using finch.

components/spellcheck/browser/spellcheck_platform_android.cc

 // 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.
 
 #include "components/spellcheck/browser/spellcheck_platform.h"
 
 #include "base/callback.h"
 #include "base/command_line.h"
-#include "components/spellcheck/common/spellcheck_switches.h"
+#include "components/spellcheck/common/spellcheck_features.h"
 
 namespace spellcheck_platform {
 
 void GetAvailableLanguages(std::vector<std::string>* spellcheck_languages) {
 }
 
 std::string GetSpellCheckerLanguage() {
   return std::string();
 }
 
 bool SpellCheckerAvailable() {
-  return base::CommandLine::ForCurrentProcess()->HasSwitch(
-      spellcheck::switches::kEnableAndroidSpellChecker);
+  return spellcheck::IsAndroidSpellCheckFeatureEnabled();
 }
 
 bool SpellCheckerProvidesPanel() {
   return false;
 }
 
 bool SpellingPanelVisible() {
   return false;
 }
 
@@ skipping 34 matching lines @@
 void CloseDocumentWithTag(int tag) {
 }
 
 void RequestTextCheck(int document_tag,
                       const base::string16& text,
                       TextCheckCompleteCallback callback) {
 
 }
 
 }  // namespace spellcheck_platform