[webcrypto] exportKey() now returns a dictionary when format='jwk' (CL 2 of 2).

Previously it would return an array buffer of the JSON.

This corresponds with the recent spec update:
https://dvcs.w3.org/hg/webcrypto-api/rev/f61017a76a5d

BUG=373917,245025

Review URL: https://codereview.chromium.org/338993002

git-svn-id: svn://svn.chromium.org/blink/trunk@176371 bbb929c8-8fbe-4397-9dbb-9b2b20218538