Return unique_ptr from WebClient::CreateWebMainParts()

ios/web/shell/shell_web_client.mm

Index: ios/web/shell/shell_web_client.mm
diff --git a/ios/web/shell/shell_web_client.mm b/ios/web/shell/shell_web_client.mm
index 825e9d1f2fb96dc8dbf3b87cb322d693d1555d38..81f10fb3c0d0718d907b29bf1a9dd1aaa061549b 100644
--- a/ios/web/shell/shell_web_client.mm
+++ b/ios/web/shell/shell_web_client.mm
@@ -6,6 +6,7 @@
 
 #import <UIKit/UIKit.h>
 
+#include "base/memory/ptr_util.h"
 #include "ios/web/public/user_agent.h"
 #include "ios/web/shell/shell_web_main_parts.h"
 
@@ -21,9 +22,11 @@ ShellWebClient::ShellWebClient() {
 ShellWebClient::~ShellWebClient() {
 }
 
-WebMainParts* ShellWebClient::CreateWebMainParts() {
-  web_main_parts_.reset(new ShellWebMainParts);
-  return web_main_parts_.get();
+std::unique_ptr<WebMainParts> ShellWebClient::CreateWebMainParts() {
+  std::unique_ptr<ShellWebMainParts> parts =
+      base::MakeUnique<ShellWebMainParts>();
+  web_main_parts_ = parts.get();
+  return std::move(parts);
 }
 
 ShellBrowserState* ShellWebClient::browser_state() const {