Limit HTMLKeygenElement.keytype to only known values.

The only supported value for the <keygen/> element's keytype attribute
is the keyword for the RSA state ("rsa".) Impose the required
normalization of the IDL attribute when getting its value.

R=
BUG=321862

Review URL: https://codereview.chromium.org/123833005

git-svn-id: svn://svn.chromium.org/blink/trunk@165027 bbb929c8-8fbe-4397-9dbb-9b2b20218538