MOAR debugging: Print stdout,stderr line by line

TBR=epoger
BUG=skia:1329

Review URL: https://codereview.chromium.org/196133010

scripts/run_cmd.py

diff --git a/scripts/run_cmd.py b/scripts/run_cmd.py
index 88f5921..18182c5 100755
--- a/scripts/run_cmd.py
+++ b/scripts/run_cmd.py
@@ -185,8 +185,9 @@
   Returns:
       A dictionary with stdout, stderr, and returncode as keys.
   """
-  print popen.stdout.read()
-  print popen.stderr.read()
+  for fd in (popen.stdout, popen.stderr):
+    for line in iter(fd.readline, ''):
+      print line
   stdout, stderr = popen.communicate()
   return CommandResults.make(stdout=stdout,
                              stderr=stderr,