[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 blink-side change:
https://codereview.chromium.org/184453005/

BUG=245025

Review URL: https://codereview.chromium.org/187103003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256025 0039d316-1c4b-4281-b951-d872f2087c98
2 files changed