Fix StrictTypeChecking on interface type attributes

'undefined'/'null' would be allowed even though they shouldn't (unless
the attribute is marked as nullable.)

Also attempts to simplify the signature required for implementing the
nullable version of a interface type attribute.

Adjust CanvasRenderingContext2D.currentTransform,
HTMLMediaElement.controller and AudioBufferSourceNode.buffer slightly.

BUG=249598

Review URL: https://codereview.chromium.org/226073003

git-svn-id: svn://svn.chromium.org/blink/trunk@171043 bbb929c8-8fbe-4397-9dbb-9b2b20218538