Improve Promises Panel UI.

Screenshots:

Promise Tracker on:
https://drive.google.com/a/google.com/file/d/0BzUsdFEY9nf_MUExbnR4c1NESzA/view?usp=sharing

Promise Tracker off:
https://drive.google.com/a/google.com/file/d/0BzUsdFEY9nf_TUs5dlAtS3MydTA/view?usp=sharing

BUG=348919

Review URL: https://codereview.chromium.org/624443006

git-svn-id: svn://svn.chromium.org/blink/trunk@183278 bbb929c8-8fbe-4397-9dbb-9b2b20218538