Remove WebContents::InsertCSS since it is unused

content/browser/web_contents/web_contents_impl.cc

Index: content/browser/web_contents/web_contents_impl.cc
diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
index a9bb2ae9a77e2693e017881f3a95994fabf76048..3bcd58f26fc6cccc069b6d6350eb5e350d67d989 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -3109,11 +3109,6 @@ void WebContentsImpl::StopFinding(StopFindAction action) {
   GetOrCreateFindRequestManager()->StopFinding(action);
 }
 
-void WebContentsImpl::InsertCSS(const std::string& css) {
-  GetMainFrame()->Send(new FrameMsg_CSSInsertRequest(
-      GetMainFrame()->GetRoutingID(), css));
-}
-
 bool WebContentsImpl::WasRecentlyAudible() {
   return audio_stream_monitor_.WasRecentlyAudible() ||
          (browser_plugin_embedder_ &&