Silence uninteresting proguard output.

ProGuard outputs boring stuff to stdout (proguard version, jar path,
etc) as well as interesting stuff (notes, warnings, etc). If stdout is
entirely boring, this method suppresses the output.

Review URL: https://codereview.chromium.org/977253002

Cr-Commit-Position: refs/heads/master@{#319183}

build/android/gyp/proguard.py

@@ -28,7 +28,31 @@ def DoProguard(options):
                   '-outjars', outjars,
                   '-libraryjars', libraryjars,
                   '@' + options.proguard_config]
-  build_utils.CheckOutput(proguard_cmd, print_stdout=True)
+  build_utils.CheckOutput(proguard_cmd, print_stdout=True,
+                          stdout_filter=FilterProguardOutput)
+
+
+def FilterProguardOutput(output):
+  '''ProGuard outputs boring stuff to stdout (proguard version, jar path, etc)
+  as well as interesting stuff (notes, warnings, etc). If stdout is entirely
+  boring, this method suppresses the output.
+  '''
+  ignore_patterns = [
+    'ProGuard, version ',
+    'Reading program jar [',
+    'Reading library jar [',
+    'Preparing output jar [',
+    '  Copying resources from program jar [',
+  ]
+  for line in output.splitlines():
+    for pattern in ignore_patterns:
+      if line.startswith(pattern):
+        break
+    else:
+      # line doesn't match any of the patterns; it's probably something worth
+      # printing out.
+      return output
+  return ''
 
 
 def main(args):