Revert 237109 "When there is a session storage namespace mismatc..."

> When there is a session storage namespace mismatch during prerender swap-in, attempt merging namespaces.
> 
> R=davidben@chromium.org, jam@chromium.org, michaeln@chromium.org, creis@chromium.org
> 
> Review URL: https://codereview.chromium.org/59823003

TBR=tburkard@chromium.org

Review URL: https://codereview.chromium.org/86393002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237132 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237132