Add a WebCore.IndexedDB.BackingStore.OpenStatus.Docs histogram

R=alecflett@chromium.org, isherman@chromium.org, jsbell@chromium.org

Review URL: https://codereview.chromium.org/72063002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235225 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235225