ServiceWorker: Add a helper to disassociate pending version from documents
Current implementation uses AssociatePendingVersionToDocuments() to
disassociate a specific pending version from documents, but actually
associating and disassociating should be done by different flow:
- Associating a pending version to documents matched with the version's
scope.
- Disassociating a pending version having a specific version id from
documents.
This CL adds a helper function for disassociating and refactors tests.
BUG=n/a
TEST=content_unittests
Review URL: https://codereview.chromium.org/
304503008
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@273518 0039d316-1c4b-4281-b951-d872f2087c98