Don't forward the focus when running content shell in headless mode

BUG=341527
R=abarth@chromium.org, sky@chromium.org

Review URL: https://codereview.chromium.org/141203007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250217 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250217