[DevTools] Cleanup DevToolsTarget and DevToolsManagerDelegate after moving to devtools_discovery.

components/devtools_http_handler/devtools_http_handler_delegate.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 COMPONENTS_DEVTOOLS_HTTP_HANDLER_DEVTOOLS_HTTP_HANDLER_DELEGATE_H_
 #define COMPONENTS_DEVTOOLS_HTTP_HANDLER_DEVTOOLS_HTTP_HANDLER_DELEGATE_H_
 
 #include <string>
 
-#include "base/files/file_path.h"
+#include "url/gurl.h"
 
 namespace devtools_http_handler {
 
 class DevToolsHttpHandlerDelegate {
  public:
   virtual ~DevToolsHttpHandlerDelegate() {}
 
   // Should return discovery page HTML that should list available tabs
   // and provide attach links.
   virtual std::string GetDiscoveryPageHTML() = 0;
 
   // Returns frontend resource data by |path|. Only used if
   // |BundlesFrontendResources| returns |true|.
   virtual std::string GetFrontendResource(const std::string& path) = 0;
+
+  // Get a thumbnail for a given page. Returns non-empty string iff we have the
+  // thumbnail.
+  virtual std::string GetPageThumbnailData(const GURL& url) = 0;
 };
 
 }  // namespace devtools_http_handler
 
 #endif  // COMPONENTS_DEVTOOLS_HTTP_HANDLER_DEVTOOLS_HTTP_HANDLER_DELEGATE_H_