[webcrypto] Change HMAC key generation length from bytes to bits.

Matches the upstream change in the spec:
https://dvcs.w3.org/hg/webcrypto-api/rev/da045968abe0

Corresponding chromium-side change:
https://codereview.chromium.org/187103003/

BUG=245025

Review URL: https://codereview.chromium.org/184453005

git-svn-id: svn://svn.chromium.org/blink/trunk@168869 bbb929c8-8fbe-4397-9dbb-9b2b20218538