Remove references to specific WebCrypto section number in code comments.

The numbering has changed in the latest version of the draft, so best not to hardcode it.

BUG=245025

Review URL: https://chromiumcodereview.appspot.com/22727002

git-svn-id: svn://svn.chromium.org/blink/trunk@155870 bbb929c8-8fbe-4397-9dbb-9b2b20218538