IndexedDB: Implement IDBObjectStore::openKeyCursor()

Allow "key cursors" to be opened against object stores. This is an
optimization where the "value" is not decoded, sent across IPC, and
deserialized.

This is an addition to the IDB "v1" spec requested by deveopers; there
is consensus on adding this method with Mozilla and the spec is
straightforward.

NOTE: This CL depends on http://crrev.com/23429003

BUG=279300

Review URL: https://codereview.chromium.org/23429004

git-svn-id: svn://svn.chromium.org/blink/trunk@158851 bbb929c8-8fbe-4397-9dbb-9b2b20218538