Revert "Remove unused headers from ios/web/public/web_state"

This reverts commit 480313d9.

Reason for revert: https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8901505278757132992/+/steps/compile/0/stdout

Original change's description:
> Remove unused headers from ios/web/public/web_state
> 
> This CL removes web_state_observer.h and web_state.h from
> ios/web/public/web_state which are no longer used in downlstream repo
> (downstream code was switched to use files from ios/web/public).
> 
> Bug: 942244
> Change-Id: I1480b115343735a69347578cfd0628f983c069ca
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1816807
> Commit-Queue: Yi Su <mrsuyi@chromium.org>
> Auto-Submit: Eugene But <eugenebut@chromium.org>
> Reviewed-by: Yi Su <mrsuyi@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#698929}

TBR=eugenebut@chromium.org,mrsuyi@chromium.org

Change-Id: I7f9a7453be97af8ecd702ac4392e73416ce7dc7d
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 942244
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1815914Reviewed-by: Leonard Grey <lgrey@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698938}

ios/web/public/web_state/web_state.h

+// Copyright 2019 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_WEB_PUBLIC_WEB_STATE_WEB_STATE_H_
+#define IOS_WEB_PUBLIC_WEB_STATE_WEB_STATE_H_
+
+// TODO(crbug.com/942244): Remove this header after downstream code
+// stopped including ios/web/public/web_state/web_state.h
+
+#import "ios/web/public/web_state.h"
+
+#endif  // IOS_WEB_PUBLIC_WEB_STATE_WEB_STATE_H_

ios/web/public/web_state/web_state_observer.h

+// Copyright 2014 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_WEB_PUBLIC_WEB_STATE_WEB_STATE_OBSERVER_H_
+#define IOS_WEB_PUBLIC_WEB_STATE_WEB_STATE_OBSERVER_H_
+
+// TODO(crbug.com/942244): Remove this header after downstream code
+// stopped including ios/web/public/web_state/web_state.h
+
+#include "ios/web/public/web_state_observer.h"
+
+#endif  // IOS_WEB_PUBLIC_WEB_STATE_WEB_STATE_OBSERVER_H_