Use dict.items() instead of dict.iteritems() for py3 compat

iteritems() went away in Python 3.

R=kainino@chromium.org

Bug: 941669
Change-Id: I0c107aaa35bb1df7c126485c33c3d15cb77f03fe
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/15560
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
diff --git a/generator/extract_json.py b/generator/extract_json.py
index 2cc4b3d..40312f9 100644
--- a/generator/extract_json.py
+++ b/generator/extract_json.py
@@ -24,7 +24,7 @@
 
     output_dir = sys.argv[2]
 
-    for (name, content) in files.iteritems():
+    for (name, content) in files.items():
         output_file = output_dir + os.path.sep + name
 
         directory = os.path.dirname(output_file)