[DevTools] Fix screencast button being not visible.

When moving WebInspector to Main.js, |this| and |WebInspector| became
different things, which broke the screencast code.

BUG=362077

Review URL: https://codereview.chromium.org/233293002

git-svn-id: svn://svn.chromium.org/blink/trunk@171285 bbb929c8-8fbe-4397-9dbb-9b2b20218538