Merge StreamContext into ChromeBlobStorageContext

Merge StreamContext into ChromeBlobStorageContext

We're trying to have Blob and Stream share the same code as much as
possible. They're both storage classes with backing store but with
different semantics.

There we see much duplication of code between these context classes.

BUG=169957

[kinuko, 2013-06-25 14:45:15]

(Drive-by) this certainly reduces some duplicated code, but I'm not very sure
merging things at this layer (but not in the rest of code) helps things much. 
Are we eventually merge everything into one backend code (like: stream is a blob
principal) or will keep two separate 'registry' (blob and stream) distinct from
each other, but just share some BlobRegistry and IPC interface?

[tyoshino (SeeGerritForStatus), 2013-06-26 04:19:12]

Thanks for the comment.

The argument that we shouldn't proceed merging everything at this
point makes sense. If the semantics of Stream and Blob will diverge
much than we suppose now, we'll again need to branch them than
sharing code.

Regarding these context classes, I'm not worrying much about divergence.
It looks for me it's not likely that their fundamental roles (HTML5
interface to some backing store backed storage) change much.

Similarly, I think it makes sense that stream is dispatched by
FileAPIMessageFilter. About IPC, we should stop and think (and discuss
more at spec wg) before adding more Stream methods to BlobHostMsg.

[tyoshino (SeeGerritForStatus), 2013-10-02 04:42:27]

Abandoned