Clean up includes in testing/test_support.h

Change-Id: Id558c36d10501ce488717045bbf00bfed32e4b92
Reviewed-on: https://pdfium-review.googlesource.com/6870
Commit-Queue: dsinclair <dsinclair@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
diff --git a/samples/pdfium_test.cc b/samples/pdfium_test.cc
index 14aa6c1..a53edb7 100644
--- a/samples/pdfium_test.cc
+++ b/samples/pdfium_test.cc
@@ -7,6 +7,7 @@
 #include <stdlib.h>
 #include <string.h>
 
+#include <iterator>
 #include <map>
 #include <sstream>
 #include <string>