WebCrypto: allow AlgorithmIdentifiers to be strings (and not just dictionaries).

      This is described by the WebCrypto spec:

https://dvcs.w3.org/hg/webcrypto-api/raw-file/tip/spec/Overview.html#dfn-AlgorithmIdentifier

And used during algorithm normalization:

https://dvcs.w3.org/hg/webcrypto-api/raw-file/tip/spec/Overview.html#dfn-normalize-an-algorithm


This first changelist is not completely spec compliant, as it recognizes more than just DOMStrings. Will address that in a follow-up change.

Testing: Some of the existing tests have been modified to use the shorthand notation for AlgorithmIdentifier (both when inside a dictionary, and part of the method call).

BUG=437066

Review URL: https://codereview.chromium.org/765623002

git-svn-id: svn://svn.chromium.org/blink/trunk@186323 bbb929c8-8fbe-4397-9dbb-9b2b20218538