IndexedDB: Implement IDBCursor.continuePrimaryKey()

This is a proposed Indexed DB "v2" feature, hidden behind the
"experimental features" flag. Discussion of the proposed API
is taking place in the W3C bug tracker at:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=20257

BUG=284830

Review URL: https://codereview.chromium.org/23834004

git-svn-id: svn://svn.chromium.org/blink/trunk@163810 bbb929c8-8fbe-4397-9dbb-9b2b20218538