Re-land r246921: "Allow running content-shell headless with --dump-render-tree."

This change reverts r247148 and re-lands r246921. It's unclear if there
really is something wrong with this patch, and the crashes in
run-webkit-tests were addressed in Blink r165816.

TBR=jochen@chromium.org, sadrul@chromium.org
BUG=316712

Review URL: https://codereview.chromium.org/132903012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247278 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247278