Adds remote devtools support to cast_shell.

This is based largely on ShellDevtoolsDelegate, but is enabled/disabled
conditionally based on a preference value that can be controlled by the
cast receiver.

R=damienv@chromium.org,lcwu@chromium.org,byungchul@chromium.org
BUG=400919

Review URL: https://codereview.chromium.org/454063004

Cr-Commit-Position: refs/heads/master@{#289812}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289812 0039d316-1c4b-4281-b951-d872f2087c98
15 files changed