[App Shell] Add dev tools for debugging

BUG=None
TEST=launch app_shell with flag "--remote-debugging-port=9222", then open
"localhost:9222" in browser and see the console.

Review URL: https://codereview.chromium.org/147993009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251351 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251351