Add a content_shell flag to turn on partial site isolation.

content/shell/common/shell_content_client.h

 // Copyright (c) 2012 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 CONTENT_SHELL_COMMON_SHELL_CONTENT_CLIENT_H_
 #define CONTENT_SHELL_COMMON_SHELL_CONTENT_CLIENT_H_
 
 #include <string>
 #include <vector>
 
 #include "base/compiler_specific.h"
 #include "content/public/common/content_client.h"
 
 namespace content {
 
 std::string GetShellUserAgent();
 
 class ShellContentClient : public ContentClient {
  public:
   ~ShellContentClient() override;
 
   std::string GetUserAgent() const override;
   base::string16 GetLocalizedString(int message_id) const override;
   base::StringPiece GetDataResource(
       int resource_id,
       ui::ScaleFactor scale_factor) const override;
   base::RefCountedStaticMemory* GetDataResourceBytes(
       int resource_id) const override;
   gfx::Image& GetNativeImageNamed(int resource_id) const override;
+  bool IsSupplementarySiteIsolationModeEnabled() override;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_SHELL_COMMON_SHELL_CONTENT_CLIENT_H_