[content shell] delay closing stderr during layout tests as long as possible

BUG=307214
R=dpranke@chromium.org, dominicc@chromium.org

Review URL: https://codereview.chromium.org/27473002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229070 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229070