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Fixes issues with EWDTAH lifetime. Now EWDTAH lives as long as there is a scoped_refptr referencing it or the worker is alive.

Split off https://codereview.chromium.org/349033009/

BUG=389454

Review URL: https://codereview.chromium.org/405603002

Cr-Commit-Position: refs/heads/master@{#288362}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288362 0039d316-1c4b-4281-b951-d872f2087c98