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/OutputESSL.h b/src/compiler/translator/OutputESSL.h
index 813f1e9..c5a9634 100644
--- a/src/compiler/translator/OutputESSL.h
+++ b/src/compiler/translator/OutputESSL.h
@@ -21,7 +21,8 @@
                 bool forceHighp);
 
 protected:
-    virtual bool writeVariablePrecision(TPrecision precision);
+  bool writeVariablePrecision(TPrecision precision) override;
+
 private:
     bool mForceHighp;
 };