metrics: fix -Winconsistent-missing-override warning

BUG=chromium:453069
TEST=FEATURES="test" emerge-amd64-generic metrics with new clang.

Change-Id: I1576200c94a01af46dd2f36e87806b8c93de7b86
Reviewed-on: https://chromium-review.googlesource.com/244052
Reviewed-by: Dan Erat <derat@chromium.org>
Commit-Queue: Yunlian Jiang <yunlian@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>

metrics/metrics_library.h

@@ -59,7 +59,7 @@ class MetricsLibrary : public MetricsLibraryInterface {
   // recommended to keep this number low (e.g., 50 is normal, while
   // 100 is high).
   bool SendToUMA(const std::string& name, int sample,
-                 int min, int max, int nbuckets);
+                 int min, int max, int nbuckets) override;
 
   // Sends linear histogram data to Chrome for transport to UMA and
   // returns true on success. This method results in the equivalent of