[Extensions] Reduce string copy in renderer/ UserScript.

extensions/renderer/script_injection.cc

 // Copyright 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.
 
 #include "extensions/renderer/script_injection.h"
 
 #include <map>
 #include <utility>
 
 #include "base/lazy_instance.h"
@@ skipping 294 matching lines @@
   // asynchronously, and we should run it.
   if (!async_completion_callback_.is_null()) {
     injector_->OnInjectionComplete(std::move(execution_result_), run_location_,
                                    render_frame_);
     // Warning: this object can be destroyed after this line!
     async_completion_callback_.Run(this);
   }
 }
 
 void ScriptInjection::InjectCss() {
-  std::vector<std::string> css_sources =
+  std::vector<blink::WebString> css_sources =
       injector_->GetCssSources(run_location_);
   blink::WebLocalFrame* web_frame = render_frame_->GetWebFrame();
-  for (const std::string& css : css_sources)
-    web_frame->document().insertStyleSheet(blink::WebString::fromUTF8(css));
+  for (const blink::WebString& css : css_sources)
+    web_frame->document().insertStyleSheet(css);
 }
 
 }  // namespace extensions