Implement unprefixed EME APIs that match the latest draft

Update the API for WebContentDecryptionModule and
WebContentDecryptionModuleSession in order to implement
the latest EME API spec.

BUG=224786
TEST=Adding to API, no code using it. All existing EME tests pass.

Review URL: https://codereview.chromium.org/642583003

git-svn-id: svn://svn.chromium.org/blink/trunk@183627 bbb929c8-8fbe-4397-9dbb-9b2b20218538