Is this internal or external sdcard that you are mentioning?

Also, depending on the version of Android running on the device, it could require explicit permissions.

What version of Android are you using?

I spent a while debugging essentially this problem Friday, and came up with a solution today.  I'll upload a CL shortly.

I drafted a CL to address the Android M permissions issue:
https://codereview.chromium.org/1961943003

Thanks Paul, your CL fixes this issue. I was thinking about this might be related to permission but it's weird that logcat does not print any error log.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6f4a1e97c63ae7307010f1059948f1ffb4bb8cba

commit 6f4a1e97c63ae7307010f1059948f1ffb4bb8cba
Author: pauljensen <pauljensen@chromium.org>
Date: Wed May 11 14:36:24 2016

[Cronet] Change sample app to save netlog to app's files directory

Previously it saved to the external storage directory which
doesn't work without changes in Android Marshmallow.
I proposed addressing this in http://crrev.com/1961943003
but it greatly complicated the sample app and saving a netlog,
potentially containing private data, into a directory visible by
all apps shouldn't be encouraged.

BUG= 610174 
R=xunjieli

Review-Url: https://codereview.chromium.org/1971643002
Cr-Commit-Position: refs/heads/master@{#392918}

[modify] https://crrev.com/6f4a1e97c63ae7307010f1059948f1ffb4bb8cba/components/cronet/android/sample/AndroidManifest.xml
[modify] https://crrev.com/6f4a1e97c63ae7307010f1059948f1ffb4bb8cba/components/cronet/android/sample/src/org/chromium/cronet_sample_apk/CronetSampleActivity.java

Okay, no permissions should be required now.  The netlog can be pulled from the device via:

  adb root
  adb pull /data/data/org.chromium.cronet_sample_apk/cache/netlog.json