When there is a session storage namespace mismatch during prerender swap-in, attempt merging namespaces.
Re-submission of https://codereview.chromium.org/59823003/ see other lgtm's there

Review URL: https://codereview.chromium.org/86593004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237203 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237203