Implement complete set of `BlobPart` and related object construction

[izgzhen]

NOTE: This is assigned to myself.

Spec: https://w3c.github.io/FileAPI/#constructorBlob

Specifically, we should add support for ArrayBuffer and ArrayBufferView. The FAIL cases in test suite should be considered.

Related to issue #10778.

[KiChjang]

Support for ArrayBuffer and ArrayBufferView requires a working implementation of CustomAutoRooter. @jdm may know more about the details for this.

[izgzhen]

I read #7676 and servo/rust-mozjs#225 (Although I don't really grasp what has been done). But I'd love to know what is blocking the road and move it if necessary :)

[KiChjang]

This is the relevant mailing list topic: https://groups.google.com/forum/#!topic/mozilla.dev.servo/tqqwgAmhp8o