[Refactor] Replace static casts to WebContentsImpl with member function.

content/browser/site_per_process_browsertest.h

 // Copyright (c) 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 CONTENT_BROWSER_SITE_PER_PROCESS_BROWSERTEST_H_
 #define CONTENT_BROWSER_SITE_PER_PROCESS_BROWSERTEST_H_
 
 #include <string>
 
+#include "content/browser/web_contents/web_contents_impl.h"
 #include "content/public/test/content_browser_test.h"
+#include "content/shell/browser/shell.h"
 #include "content/test/content_browser_test_utils_internal.h"
 #include "url/gurl.h"
 
 namespace content {
 
 class FrameTreeNode;
-class Shell;
 
 class SitePerProcessBrowserTest : public ContentBrowserTest {
  public:
   SitePerProcessBrowserTest();
 
  protected:
   std::string DepictFrameTree(FrameTreeNode* node);
 
   void SetUpCommandLine(base::CommandLine* command_line) override;
   void SetUpOnMainThread() override;
 
+  WebContentsImpl* web_contents() const {
+    return static_cast<WebContentsImpl*>(shell()->web_contents());
+  }
+
  private:
   FrameTreeVisualizer visualizer_;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_SITE_PER_PROCESS_BROWSERTEST_H_