[webcrypto] Remove the TODO for NullKey().

BUG=245025

Review URL: https://codereview.chromium.org/53533004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232917 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232917