Make various string_util functions take StringPieces instead of char[].

chrome/browser/guest_view/ad_view/ad_view_guest.cc

Index: chrome/browser/guest_view/ad_view/ad_view_guest.cc
diff --git a/chrome/browser/guest_view/ad_view/ad_view_guest.cc b/chrome/browser/guest_view/ad_view/ad_view_guest.cc
index dd6b586cd42c45ba8f4dcb6ce384baedd15bf699..2e5106b6ee251d6dcec292cbdba363ef3ac6dd01 100644
--- a/chrome/browser/guest_view/ad_view/ad_view_guest.cc
+++ b/chrome/browser/guest_view/ad_view/ad_view_guest.cc
@@ -50,8 +50,9 @@ void AdViewGuest::DidFailProvisionalLoad(
     const base::string16& error_description,
     content::RenderViewHost* render_view_host) {
   // Translate the |error_code| into an error string.
-  std::string error_type;
-  base::RemoveChars(net::ErrorToString(error_code), "net::", &error_type);
+  std::string error_type(net::ErrorToString(error_code));
+  DCHECK(StartsWithASCII(error_type, "net::", true));
+  error_type.erase(0, 5);
 
   scoped_ptr<base::DictionaryValue> args(new base::DictionaryValue());
   args->SetBoolean(guestview::kIsTopLevel, is_main_frame);