[FileAPI] Deterministic release of Blob proposal

At TPAC we discussed the ability to deterministically close blobs with a few
others.

As we've discussed in the createObjectURL thread[1], a Blob may represent
an expensive resource (eg. expensive in terms of memory, battery, or disk
space). At present there is no way for an application to deterministically
release the resource backing the Blob. Instead, an application must rely on
the resource being cleaned up through a non-deterministic garbage collector
once all references have been released. We have found that not having a way
to deterministically release the resource causes a performance impact for a
certain class of applications, and is especially important for mobile applications
or devices with more limited resources.

In particular, we've seen this become a problem for media intensive applications
which interact with a large number of expensive blobs. For example, a gallery
application may want to cycle through displaying many large images downloaded
through websockets, and without a deterministic way to immediately release
the reference to each image Blob, can easily begin to consume vast amounts of
resources before the garbage collector is executed.

To address this issue, we propose that a close method be added to the Blob
interface.
When called, the close method should release the underlying resource of the
Blob, and future operations on the Blob will return a new error, a ClosedError.
This allows an application to signal when it's finished using the Blob.

To support this change, the following changes in the File API spec are needed:

* In section 6 (The Blob Interface)
  - Addition of a close method. When called, the close method releases the
underlying resource of the Blob. Close renders the blob invalid, and further
operations such as URL.createObjectURL or the FileReader read methods on
the closed blob will fail and return a ClosedError.  If there are any non-revoked
URLs to the Blob, these URLs will continue to resolve until they have been
revoked.
  - For the slice method, state that the returned Blob is a new Blob with its own
lifetime semantics - calling close on the new Blob is independent of calling close
on the original Blob.

*In section 8 (The FIleReader Interface)
- State the FileReader reads directly over the given Blob, and not a copy with
an independent lifetime.

* In section 10 (Errors and Exceptions)
- Addition of a ClosedError. If the File or Blob has had the close method called,
then for asynchronous read methods the error attribute MUST return a
"ClosedError" DOMError and synchronous read methods MUST throw a
ClosedError exception.

* In section 11.8 (Creating and Revoking a Blob URI)
- For createObjectURL - If this method is called with a closed Blob argument,
then user agents must throw a ClosedError exception.

Similarly to how slice() clones the initial Blob to return one with its own
independent lifetime, the same notion will be needed in other APIs which
conceptually clone the data - namely FormData, any place the Structured Clone
Algorithm is used, and BlobBuilder.
Similarly to how FileReader must act directly on the Blob's data, the same notion
will be needed in other APIs which must act on the data - namely XHR.send and
WebSocket. These APIs will need to throw an error if called on a Blob that was
closed and the resources are released.

We've recently implemented this in experimental builds and have seen measurable
performance improvements.

The feedback we heard from our discussions with others at TPAC regarding our
proposal to add a close() method to the Blob interface was that objects in the web
platform potentially backed by expensive resources should have a deterministic
way to be released.

Thanks,
Feras

[1] http://lists.w3.org/Archives/Public/public-webapps/2011OctDec/1499.html