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

components/spellcheck/common/spellcheck_features.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 COMPONENTS_SPELLCHECK_COMMON_SPELLCHECK_FEATURES_H_
+#define COMPONENTS_SPELLCHECK_COMMON_SPELLCHECK_FEATURES_H_
+
+#include "base/feature_list.h"
+
+namespace spellcheck {
+
+#if defined(ENABLE_SPELLCHECK) && defined(OS_ANDROID)
+extern const base::Feature kAndroidSpellChecker;
+extern const base::Feature kAndroidSpellCheckerNonLowEnd;
+
+bool IsAndroidSpellCheckFeatureEnabled();
+#endif  // defined(ENABLE_SPELLCHECK) && defined(OS_ANDROID)
+
+}  // namespace spellcheck
+
+#endif  // COMPONENTS_SPELLCHECK_COMMON_SPELLCHECK_FEATURES_H_