Standardize usage of virtual/override/final in net/

This patch was automatically generated by applying clang fixit hints
generated by the plugin to the source tree.

BUG=417463
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Review URL: https://codereview.chromium.org/667923003

Cr-Commit-Position: refs/heads/master@{#300466}
diff --git a/net/disk_cache/blockfile/backend_impl_v3.cc b/net/disk_cache/blockfile/backend_impl_v3.cc
index 09794d34..a1024b4 100644
--- a/net/disk_cache/blockfile/backend_impl_v3.cc
+++ b/net/disk_cache/blockfile/backend_impl_v3.cc
@@ -1510,8 +1510,8 @@
 
 class BackendImplV3::NotImplementedIterator : public Backend::Iterator {
  public:
-  virtual int OpenNextEntry(disk_cache::Entry** next_entry,
-                            const net::CompletionCallback& callback) override {
+  int OpenNextEntry(disk_cache::Entry** next_entry,
+                    const net::CompletionCallback& callback) override {
     return net::ERR_NOT_IMPLEMENTED;
   }
 };