GitLab

This CL includes:
- Replace custom progress bar with paper-slider.
- Pause track temporalily while dragging progress bar to be consistent with video player.
- Adjust layout of control panel.
- Show playing time as "{current} / {duration}".

BUG=488229
TEST=manually

Review URL: https://codereview.chromium.org/1403303011

Cr-Commit-Position: refs/heads/master@{#357064}