Standardize usage of virtual/override/final specifiers in base/.

base/memory/discardable_memory_shmem.h

Index: base/memory/discardable_memory_shmem.h
diff --git a/base/memory/discardable_memory_shmem.h b/base/memory/discardable_memory_shmem.h
index d25f84cb46dbdb364d1a86dbf4c9286c8759d02e..c2ab27849f290fbdfffe163f020418f376e4b8c0 100644
--- a/base/memory/discardable_memory_shmem.h
+++ b/base/memory/discardable_memory_shmem.h
@@ -19,7 +19,7 @@ class DiscardableMemoryShmem
       public internal::DiscardableMemoryManagerAllocation {
  public:
   explicit DiscardableMemoryShmem(size_t bytes);
-  virtual ~DiscardableMemoryShmem();
+  ~DiscardableMemoryShmem() override;
 
   static void ReleaseFreeMemory();
 
@@ -28,15 +28,15 @@ class DiscardableMemoryShmem
   bool Initialize();
 
   // Overridden from DiscardableMemory:
-  virtual DiscardableMemoryLockStatus Lock() override;
-  virtual void Unlock() override;
-  virtual void* Memory() const override;
+  DiscardableMemoryLockStatus Lock() override;
+  void Unlock() override;
+  void* Memory() const override;
 
   // Overridden from internal::DiscardableMemoryManagerAllocation:
-  virtual bool AllocateAndAcquireLock() override;
-  virtual void ReleaseLock() override;
-  virtual void Purge() override;
-  virtual bool IsMemoryResident() const override;
+  bool AllocateAndAcquireLock() override;
+  void ReleaseLock() override;
+  void Purge() override;
+  bool IsMemoryResident() const override;
 
  private:
   const size_t bytes_;