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/Compiler.h b/src/compiler/translator/Compiler.h
index d1f2912..1fb5c82 100644
--- a/src/compiler/translator/Compiler.h
+++ b/src/compiler/translator/Compiler.h
@@ -67,8 +67,8 @@
 {
   public:
     TCompiler(sh::GLenum type, ShShaderSpec spec, ShShaderOutput output);
-    virtual ~TCompiler();
-    virtual TCompiler* getAsCompiler() { return this; }
+    ~TCompiler() override;
+    TCompiler *getAsCompiler() override { return this; }
 
     bool Init(const ShBuiltInResources& resources);