[Extensions] Reduce string copy in renderer/ UserScript.

extensions/renderer/script_injection.cc

Index: extensions/renderer/script_injection.cc
diff --git a/extensions/renderer/script_injection.cc b/extensions/renderer/script_injection.cc
index da93590c3908c00fb33421a2b3ff9f1077e04b06..69cfc526c797f2cd7b9407495b34290765210d9e 100644
--- a/extensions/renderer/script_injection.cc
+++ b/extensions/renderer/script_injection.cc
@@ -312,11 +312,11 @@ void ScriptInjection::OnJsInjectionCompleted(
 }
 
 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