Add webkitOfflineAudioContext definition, and remove invalid constructor.

    tsc --noImplicitAny webaudioapi/waa-tests.ts

and

    tsc --noImplicitAny webaudioapi/waa.d.ts

occur compile error.

> webaudioapi/waa.d.ts(183,5): error TS7010: 'constructor', which lacks return-type annotation, implicitly has an 'any' return type.

So, added `webkitOfflineAudioContext` definition and `new` method, and remove `constructor` from `OfflineAudioContext` interface.