DevTools: move non panel-specific CSS from layerPanel.css to timelinePanel.css

... so we can match the direction of code dependency (Layers panel currently
depends on Timeline panel to show shared layer views).

BUG=

Review URL: https://codereview.chromium.org/658793002

git-svn-id: svn://svn.chromium.org/blink/trunk@183758 bbb929c8-8fbe-4397-9dbb-9b2b20218538