DevTools: CSS cleanup.

Source/devtools/front_end/inspectorStyle.css:1891 hide-disclosure-buttons
Never used.

Source/devtools/front_end/timelinePanel.css:358 timeline-loading
There is no aggregates stats in popover anymore.

Source/devtools/front_end/elementsPanel.css:648 css-font-value
https://codereview.chromium.org/59993003

Source/devtools/front_end/inspectorStyle.css:1821 console-filter-top
https://bugs.webkit.org/show_bug.cgi?id=68155

BUG=

Review URL: https://codereview.chromium.org/471873002

git-svn-id: svn://svn.chromium.org/blink/trunk@180268 bbb929c8-8fbe-4397-9dbb-9b2b20218538