[webcrypto] Implicitly set public keys to extractable.

There was consensus on this issue during TPAC, and this behavior will be called out in a forthcoming spec update.

See also: https://www.w3.org/Bugs/Public/show_bug.cgi?id=23695

BUG=245025

Review URL: https://codereview.chromium.org/104923004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239109 0039d316-1c4b-4281-b951-d872f2087c98
3 files changed