DevTools: Remove few jsdocs casts.

Improves compiler coverage, fixes few jsdocs bugs.

BUG=None
R=vsevik@chromium.org

Review URL: https://codereview.chromium.org/22438002

git-svn-id: svn://svn.chromium.org/blink/trunk@155753 bbb929c8-8fbe-4397-9dbb-9b2b20218538