Adds a new flag for mailto handling.

A new mailto handler provider is being added to Chrome for iOS. This flag will
wrap all invocations of the new provider during development.

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I9e24a5f256fc5e220120e173080a67301084b506
Reviewed-on: https://chromium-review.googlesource.com/876729Reviewed-by: Peter Lee <pkl@chromium.org>
Commit-Queue: Daniel Blakemore <danblakemore@google.com>
Cr-Commit-Position: refs/heads/master@{#532980}

ios/chrome/browser/mailto/BUILD.gn

+# Copyright 2018 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.
+
+source_set("features") {
+  configs += [ "//build/config/compiler:enable_arc" ]
+  sources = [
+    "features.h",
+    "features.mm",
+  ]
+  deps = [
+    "//base",
+  ]
+}

ios/chrome/browser/mailto/features.h

+// Copyright 2018 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 IOS_CHROME_BROWSER_MAILTO_FEATURES_H_
+#define IOS_CHROME_BROWSER_MAILTO_FEATURES_H_
+
+#include "base/feature_list.h"
+
+// Feature flag to enable Mailto handling.
+extern const base::Feature kMailtoHandledWithGoogleUI;
+
+#endif  // IOS_CHROME_BROWSER_MAILTO_FEATURES_H_

ios/chrome/browser/mailto/features.mm

+// Copyright 2018 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 "ios/chrome/browser/mailto/features.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+const base::Feature kMailtoHandledWithGoogleUI{
+    "kMailtoHandledWithGoogleUI", base::FEATURE_DISABLED_BY_DEFAULT};