IndexedDB: Deflake a couple of layout tests

These two tests - clone-exception.html and connection-leak.html - were
not doing the regular "delete before creating" dance, which caused them
to timeout flakily when run in repeated batches. Adding a deletion step
causes the flakiness to disappear.

BUG=374878
TBR=cmumford@chromium.org

Review URL: https://codereview.chromium.org/294723002

git-svn-id: svn://svn.chromium.org/blink/trunk@174312 bbb929c8-8fbe-4397-9dbb-9b2b20218538