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

content/browser/site_per_process_browsertest.h

Index: content/browser/site_per_process_browsertest.h
diff --git a/content/browser/site_per_process_browsertest.h b/content/browser/site_per_process_browsertest.h
index 16db097ed49ecb0bee58f427169f791f31003ef5..32861ec6d3978e1139d0bd3201b34c1e2aa1f9b7 100644
--- a/content/browser/site_per_process_browsertest.h
+++ b/content/browser/site_per_process_browsertest.h
@@ -7,14 +7,15 @@
 
 #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:
@@ -26,6 +27,10 @@ class SitePerProcessBrowserTest : public ContentBrowserTest {
   void SetUpCommandLine(base::CommandLine* command_line) override;
   void SetUpOnMainThread() override;
 
+  WebContentsImpl* web_contents() const {
+    return static_cast<WebContentsImpl*>(shell()->web_contents());
+  }
+
  private:
   FrameTreeVisualizer visualizer_;
 };