Navigate first tab to about:blank as a workaround to get dromaeo tests running on cros.

This also fixes tough_media_cases.

BUG=329271
TEST=local runs of dromaeo
NOTRY=True

Review URL: https://codereview.chromium.org/108583006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241763 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241763