[ios] Moves find-in-page code out of Tab and into FindTabHelper.

ios/chrome/browser/find_in_page/find_tab_helper_unittest.mm

+// Copyright 2017 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.
+
+#import "ios/chrome/browser/find_in_page/find_tab_helper.h"
+
+#include "base/macros.h"
+#import "ios/web/public/test/fakes/test_web_state.h"
+#import "ios/web/public/test/web_test.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+// Test fixture for the FindTabHelper class.
+class FindTabHelperTest : public web::WebTest {
+ public:
+  FindTabHelperTest() { FindTabHelper::CreateForWebState(&web_state_, nil); }
+  ~FindTabHelperTest() override = default;
+
+ protected:
+  web::TestWebState web_state_;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(FindTabHelperTest);
+};
+
+// Tests that the helper's FindInPageController exists.
+TEST_F(FindTabHelperTest, ControllerExists) {
+  DCHECK(FindTabHelper::FromWebState(&web_state_)->GetController());
+}