Don't terminate SharedWorker while loading the script.

This path will fix the heap-use-after-free bug.

BUG=343661,344750

Review URL: https://codereview.chromium.org/171943002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252010 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252010