DevTools: fix styles for glasspane

The glasspane should be a component-root so that it inherits all the
basic styling.

R=pfeldman

Review URL: https://codereview.chromium.org/659043002

git-svn-id: svn://svn.chromium.org/blink/trunk@183813 bbb929c8-8fbe-4397-9dbb-9b2b20218538