Use override in all the places where it is possible

This will avoid -Winconsistent-overrides in the future. Done using the
-Wsuggest-override warning of GCC 5.1

BUG=

Change-Id: I707a649dc368f5dd1e139fd144370abcac0b6263
Reviewed-on: https://chromium-review.googlesource.com/294920
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/compiler/translator/FlagStd140Structs.h b/src/compiler/translator/FlagStd140Structs.h
index 53630c8..cfcd775 100644
--- a/src/compiler/translator/FlagStd140Structs.h
+++ b/src/compiler/translator/FlagStd140Structs.h
@@ -27,8 +27,8 @@
     const std::vector<TIntermTyped *> getFlaggedNodes() const { return mFlaggedNodes; }
 
   protected:
-    virtual bool visitBinary(Visit visit, TIntermBinary *binaryNode);
-    virtual void visitSymbol(TIntermSymbol *symbol);
+    bool visitBinary(Visit visit, TIntermBinary *binaryNode) override;
+    void visitSymbol(TIntermSymbol *symbol) override;
 
   private:
     bool isInStd140InterfaceBlock(TIntermTyped *node) const;