WebCrypto: Add more operations to the platform API.

Adds methods for encrypt(), decrypt() and sign()

BUG=245025

Review URL: https://chromiumcodereview.appspot.com/21016005

git-svn-id: svn://svn.chromium.org/blink/trunk@155224 bbb929c8-8fbe-4397-9dbb-9b2b20218538