DevTools: nits: introduce a handy asBool(bool*) and use across agents.

R=loislo, yurys

Review URL: https://codereview.chromium.org/427803002

git-svn-id: svn://svn.chromium.org/blink/trunk@179157 bbb929c8-8fbe-4397-9dbb-9b2b20218538