IndexedDB: Support binary keys (experimental feature) (4/4)

Enable binary key support when the "experimental web
platform features" flag is set.

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

BUG=297304

Review URL: https://codereview.chromium.org/24323009

git-svn-id: svn://svn.chromium.org/blink/trunk@165578 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Forgotten follow-up cleanups in WebFileSystem.h

Was meant to be removed soon, but has been forgotten

BUG=257349

Review URL: https://codereview.chromium.org/144633004

git-svn-id: svn://svn.chromium.org/blink/trunk@165579 bbb929c8-8fbe-4397-9dbb-9b2b20218538