ozone: gbm: Fix missing OVERRIDE

BUG=none
TEST=compile
TBR=dnicoara
NOTRY=true

Review URL: https://codereview.chromium.org/518303002

Cr-Commit-Position: refs/heads/master@{#292712}

ui/ozone/platform/dri/gbm_surface_factory.cc

@@ -28,7 +28,8 @@ class SingleOverlay : public OverlayCandidatesOzone {
   SingleOverlay() {}
   virtual ~SingleOverlay() {}
 
-  virtual void CheckOverlaySupport(OverlaySurfaceCandidateList* candidates) {
+  virtual void CheckOverlaySupport(
+      OverlaySurfaceCandidateList* candidates) OVERRIDE {
     if (candidates->size() == 2) {
       OverlayCandidatesOzone::OverlaySurfaceCandidate* first =
           &(*candidates)[0];