Make ConvolverNode.buffer and WaveShaperNode.curve nullable

These are nullable in the spec:
https://webaudio.github.io/web-audio-api/#ConvolverNode
https://webaudio.github.io/web-audio-api/#WaveShaperNode

Because of [LegacyInterfaceTypeChecking], this does not change the
generated code at all, and is therefore not testable. However, it will
result in the correct behavior once [LegacyInterfaceTypeChecking] is
removed, to be done separately.

BUG=561338

Review URL: https://codereview.chromium.org/1497823003

Cr-Commit-Position: refs/heads/master@{#363159}
2 files changed