GitLab

This CL adds a controlled-by indicator when a slider is controlled by
policy or an extention.

BUG=693302
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2737083002
Cr-Commit-Position: refs/heads/master@{#456247}