Include MediaSource overload for createObjectURL

[saschanaz]

Currently the MSE spec adds overload for createObjectURL using partial interface, but the Web IDL spec says:

Operations must not be overloaded across interface, partial interface, interface mixin, and partial interface mixin definitions.

We should directly include the overload in the original definition.

[annevk]

Another reason this is a good idea is that we don't really want to spread this pattern further. It's incredibly leaky and error-prone.

[wolenetz]

Further: "If [Exposed] appears on an overloaded operation, then it must appear identically on all overloads." per https://heycam.github.io/webidl/#Exposed (as noted by @bzbarsky w3c/media-source#168 (comment))