Enforce marking "override" for functions overriding Blink in content/renderer/.

BUG=535367
TEST=it all stays working

Review URL: https://codereview.chromium.org/1362323003

Cr-Commit-Position: refs/heads/master@{#350895}
diff --git a/content/renderer/pepper/url_response_info_util.cc b/content/renderer/pepper/url_response_info_util.cc
index 736a95c..e14f1ec 100644
--- a/content/renderer/pepper/url_response_info_util.cc
+++ b/content/renderer/pepper/url_response_info_util.cc
@@ -33,7 +33,7 @@
  public:
   const std::string& buffer() const { return buffer_; }
 
-  virtual void visitHeader(const WebString& name, const WebString& value) {
+  void visitHeader(const WebString& name, const WebString& value) override {
     if (!buffer_.empty())
       buffer_.append("\n");
     buffer_.append(name.utf8());