WebCrypto: Add WebKit API structure for keys.

This corresponds to the "Key interface" in the spec:
https://dvcs.w3.org/hg/webcrypto-api/raw-file/tip/spec/Overview.html#key-interface

This changelist introduces "WebCryptoKey" as a reference-counted wrapper to an opaque key handle.
This opaque key handle is a pointer created by the embedder, which derives from WebCryptoKeyHandle.

Testing: This change does not have any accompanying tests, as there isn't a way to create key objects from Javascript yet. These interfaces will get test coverage once key generation is hooked up.

BUG=245025

Review URL: https://chromiumcodereview.appspot.com/18033004

git-svn-id: svn://svn.chromium.org/blink/trunk@153657 bbb929c8-8fbe-4397-9dbb-9b2b20218538