Remove BlobBuilder.

The BlobBuilder JavaScript API was removed some time ago. However, its
implementation in core/fileapi/BlobBuilder.{cpp,h} remained, as it was
used by the Blob constructor.

This commit pushes the functionality from BlobBuilder into Blob, File
and BlobData.

BUG=315038

Review URL: https://codereview.chromium.org/64713007

git-svn-id: svn://svn.chromium.org/blink/trunk@164151 bbb929c8-8fbe-4397-9dbb-9b2b20218538