Add a new GetInstance() method for singleton classes under chrome/service and /net.

net/base/bandwidth_metrics.h

Index: net/base/bandwidth_metrics.h
diff --git a/net/base/bandwidth_metrics.h b/net/base/bandwidth_metrics.h
index 80fa00564a3ea65bff7375375ac0428fa9cdd136..aef366dba247473b2e4e4821fdcaec0c6cda7db7 100644
--- a/net/base/bandwidth_metrics.h
+++ b/net/base/bandwidth_metrics.h
@@ -121,9 +121,7 @@ class BandwidthMetrics {
 // ensure we always stop them.
 class ScopedBandwidthMetrics {
  public:
-  explicit ScopedBandwidthMetrics(BandwidthMetrics* metrics)
-      : metrics_(metrics), started_(false) {
-  }
+  ScopedBandwidthMetrics();
 
   ~ScopedBandwidthMetrics() {
     if (started_)