IndexedDB: Support binary keys in Chromium back-end

Back-end support for binary values (Uint8Arrays) as IDB keys.

This requires Blink public API changes before landing.

(This is part 3 of a 4 part landing sequence.)

BUG=297304

Review URL: https://codereview.chromium.org/23439009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236380 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236380