Prevent UaF in test, but still fail if the tab unexpectedly closes.

BUG=314036
TEST=BrowserTest.NoStopDuringTransferUntilCommit
R=nasko@chromium.org, sky@chromium.org

Review URL: https://codereview.chromium.org/47873008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232745 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232745
Simple Cache: Enable for media cache

BUG=

Review URL: https://codereview.chromium.org/29763005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232747 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232747