IndexedDB: Tests for cursor prefetch edge cases

Regression tests for Chromium https://codereview.chromium.org/124323002/

BUG=331570

Review URL: https://codereview.chromium.org/105213009

git-svn-id: svn://svn.chromium.org/blink/trunk@164701 bbb929c8-8fbe-4397-9dbb-9b2b20218538