Revert of [Blob] BlobReader class & tests, and removal of all redundant reading. (patchset #15 id:280001 of https://codereview.chromium.org/1337153002/ )

Reason for revert:
BlobReaderTest.StateErrors failing on multiple bots.

../../content/browser/fileapi/blob_reader_unittest.cc:985: Failure
Death test: reader_->Read(buffer.get(), 10, &bytes_read, base::Bind(&SetValue<int>, &async_bytes_read))
    Result: failed to die.
 Error msg:
[  DEATH   ]

Original issue's description:
> [Blob] BlobReader class & tests, and removal of all redundant reading.
>
> * New BlobReader class & tests
> * Removal of other reading code, which now uses the BlobReader
> * Removal of unnecessary UploadDiskCacheEntryElementReader
> * Removal of blob expansion logic in UploadDataStreamBuilder
>
> Now it's very easy for anyone in browserland to read blobs instead of
> having to do url requests, and it's also easy for anyone to add new
> blob backing storage.
>
> This is a prerequisite for the new blob async transportation, see here:
> goto/BlobPaging
>
> BUG=138051,375297
>
> Committed: https://crrev.com/02561552a57ed8792a8ebb2676bad485e1d99605
> Cr-Commit-Position: refs/heads/master@{#351470}

TBR=michaeln@chromium.org,mmenke@chromium.org,dmurph@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=138051,375297

Review URL: https://codereview.chromium.org/1376123002

Cr-Commit-Position: refs/heads/master@{#351485}
26 files changed