Fix cpplint build/namespaces issues

Change-Id: I19c68703270c1482d6c6aad8cdf97d3d2924360a
diff --git a/runtime/base/histogram_test.cc b/runtime/base/histogram_test.cc
index ea3e35f..9f3587a 100644
--- a/runtime/base/histogram_test.cc
+++ b/runtime/base/histogram_test.cc
@@ -20,7 +20,7 @@
 
 #include <sstream>
 
-using namespace art;
+namespace art {
 
 //Simple usage:
 //  Histogram *hist = new Histogram("SimplePercentiles");
@@ -266,3 +266,5 @@
   hist->PrintConfidenceIntervals(stream, 0.99);
   EXPECT_EQ(expected, stream.str());
 }
+
+} // namespace art