Show all heap snapshots in the base snapshot selector

Current heap snapshot wasn't added to the list of base snapshots of the last snapshot in the list as it was still marked as 'being recorded'. This change only adds a snapshot to the base list when its transfer is complete.

BUG=339429
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/149293003

git-svn-id: svn://svn.chromium.org/blink/trunk@166132 bbb929c8-8fbe-4397-9dbb-9b2b20218538