Remove obsolete WebString comment from string_util

base/strings/string_util.h

Index: base/strings/string_util.h
diff --git a/base/strings/string_util.h b/base/strings/string_util.h
index 31c900362d57557e10edcc73e072a5647e0e8853..e5cce8fd97e56f93e0717250b2b4bb60851c4806 100644
--- a/base/strings/string_util.h
+++ b/base/strings/string_util.h
@@ -284,8 +284,6 @@ BASE_EXPORT bool ContainsOnlyChars(const StringPiece16& input,
 BASE_EXPORT bool IsStringUTF8(const StringPiece& str);
 BASE_EXPORT bool IsStringASCII(const StringPiece& str);
 BASE_EXPORT bool IsStringASCII(const StringPiece16& str);
-// A convenience adaptor for WebStrings, as they don't convert into
-// StringPieces directly.
 BASE_EXPORT bool IsStringASCII(const string16& str);
 #if defined(WCHAR_T_IS_UTF32)
 BASE_EXPORT bool IsStringASCII(const std::wstring& str);