Expose a default constructor for WebCryptoKey.

(The motivation is code cleanup, since WebCryptoKey is instantiated in many places; blink::WebCryptoKey::createNull() is less readable)

BUG=425251

Review URL: https://codereview.chromium.org/669763003

git-svn-id: svn://svn.chromium.org/blink/trunk@184021 bbb929c8-8fbe-4397-9dbb-9b2b20218538