Suppress refcounter races when destroying IndexedDBContextImpl (issue 321596)

BUG=321596
TBR=jsbell@chromium.org

Review URL: https://codereview.chromium.org/78353002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236234 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236234