Suppress stdout in run_proguard_dx_on_app.py

The large amount of output was causing benchmarks to fail.

Change-Id: I2f2f196f56fe5e6f0f38e7e69386ea104aacad2e
diff --git a/tools/run_proguard_dx_on_app.py b/tools/run_proguard_dx_on_app.py
index a953229..4624f26 100755
--- a/tools/run_proguard_dx_on_app.py
+++ b/tools/run_proguard_dx_on_app.py
@@ -115,7 +115,10 @@
     track_memory_file = None
     if options.print_memoryuse:
       track_memory_file = join(temp, utils.MEMORY_USE_TMP_FILE)
-    proguard.run(args, track_memory_file = track_memory_file)
+    proguard.run(
+        args,
+        track_memory_file = track_memory_file,
+        stdout=open(os.devnull, 'w'))
     if options.print_memoryuse:
       proguard_memoryuse = utils.grep_memoryuse(track_memory_file)