Timeline: remove atavism of hand-crafter pre-split view implementation.

A div.timeline-sidebar-background manually added to timeline panel does
not seem to have any visual effect and apparently comes from the times
of hand-crafted split view implementation.

BUG=

Review URL: https://chromiumcodereview.appspot.com/19952003

git-svn-id: svn://svn.chromium.org/blink/trunk@155696 bbb929c8-8fbe-4397-9dbb-9b2b20218538