Add mechanism to get the Tab from the associated WebState.

ios/chrome/browser/tabs/legacy_tab_helper.mm

Index: ios/chrome/browser/tabs/legacy_tab_helper.mm
diff --git a/ios/chrome/browser/tabs/legacy_tab_helper.mm b/ios/chrome/browser/tabs/legacy_tab_helper.mm
new file mode 100644
index 0000000000000000000000000000000000000000..68544df220a2f16f21efdfa48ad9425bdb641656
--- /dev/null
+++ b/ios/chrome/browser/tabs/legacy_tab_helper.mm
@@ -0,0 +1,23 @@
+// 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/tabs/legacy_tab_helper.h"
+
+#import "ios/chrome/browser/tabs/tab.h"
+
+DEFINE_WEB_STATE_USER_DATA_KEY(LegacyTabHelper);
+
+// static
+void LegacyTabHelper::CreateForWebState(web::WebState* web_state, Tab* tab) {
+  DCHECK(web_state);
+  DCHECK(!FromWebState(web_state));
+  web_state->SetUserData(UserDataKey(), new LegacyTabHelper(web_state, tab));
+}
+
+LegacyTabHelper::~LegacyTabHelper() = default;
+
+LegacyTabHelper::LegacyTabHelper(web::WebState* web_state, Tab* tab)
+    : tab_(tab) {
+  DCHECK_EQ(web_state, tab.webState);
+}