GitLab

This CL makes the left and right arrows close and open folders in the
sidebar respectively. A closed folder will transfer selection to the
parent when left is pressed and an open folder will transfer selection
to its first child.

BUG=692844
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2857893002
Cr-Commit-Position: refs/heads/master@{#471199}